First, you are probably not working and are actually procrastinating by reading this article. So stop now and get to work! If you can't, then this article is for you, maybe it'll change how you look at things. The article aims at people who work from home or have similar freedom to decide when, where and how to work. I'm trying to mention all unconventional areas that might be on your excuses.txt :)
Physical problems or tiredness.
Eye strain. Lubricating eye droplets, rest for 10 mins.
Back strain. Re-evaluate you chair. Are you sure it's well-suited for prolonged hours of work? Try working from an alternative furniture, such as a bed or beanbag - as long as you don't fall asleep, it can provide temporary relief.
Nail problems. Biting your nails is an anxiety problem but could be eased by two solutions. First is to have a small healthy snak nearby - nuts. And eat often, in general. Otherwise find a small toy (not children's), a small object that you can play with when you are thinking.
Food. You can go for hours if you are deep into the work, but do you forget to eat? Sometimes you are lazy to conjure food, but the hunger disallows you to work productively. Set up a timer for 3 hours after your last meal so you can get more food in time to avoid this problem. Make sure you work out 2-3 times a week so those calories are put to a good use and not on your waistline. Also, if you work and eat at the same time, you eat slower and may reach fullness before finishing your food. Then the plates are in front of you, prohibiting effective working. Don't work and eat at the same time. The only time you should watch youtube videos and read articles like this, while you are eating.
Tip: Combine things you do. Multitask, if you can. You'll have more time! E.g. eating for 20 minutes then checking out daily updates from your YouTube channels or Facebook updates for another 20 minutes is not good. Combine the two, and you just saved 20 minutes.
Too much silence. Sometimes it's good to hear your thoughts and concentrate in silence. But in general, work with music. Take advantage of the fact that you can do that, certain workplaces don't let you listen to your music.
Bad weather. The 'worse' the weather is, the more likely for you to stay indoors and work. If you don't like clouds then think about the sunny sea of clouds you see from a plane or a high mountain. If anything, you could be complaining about sunshine. It reflects its way into your work spot, blindingly reminding that it's probably better outside. Go for a walk then come back to work twice as hard. Plan your steps with your steps - create an imaginary to-do while you are walking. Don't do that if you bring a companion with you.
Anything else: go work out. Has it been more than 2 days since your last work out session (any sport)? Then what are you waiting for? You'll get energized and you day will have a highlight.
Tip: Try a sauna and jacuzzi at a wellness center or similar, it can boost you for the rest of the day. The type of boost is better suited for housework like cleaning or cooking or other physical activity, but hey, that needs to get done too!
Not being able to rest well at night. Are you sure you are sleeping and at night? Try an app that measures your sleep patterns, it tells if you are unrest where you should be sleeping calmly. Also, try going to sleep earlier and waking up at a humanly time. Work is not worth messing up your sleep. Furthermore, check if your bed has the type of mattress you actually like and if it's suited to your sleeping position. If you sleep on your side, then your shoulders may get pressed against the hard mattress. I recommend a memory foam topper of a few inches. Extremely soft surfaces aren't that good if you sleep on your stomach because your face will sink into the fabric so much, you won't be able to breath through the downside nostril. Yes, first world problems :)
Tip: Push yourself. Almost literally. Get up and perform a Jim Carrey-ish movement where your arms act like rubber pool noodles and push/drag yourself in the general direction where you need to be. Like an outside force was moving you. It's very silly but at the same time is especially useful if you need to get up from the bed.
It's okay to make certain excuses
If it's not taken to the extremes, meaning you never work because you skip it all the time. The areas of life that can justify skipping work is your health and your love life. As a matter of fact you should skip work about 2/3 of the time to spend enough time with your significant other (or allocate the same amount of time to find pleasure and happiness) and to improve your health. If you become comfortable knowin that it's okay to skip work but only for these alternatives, you'll find that your time will get spent in a good way, even though you are not productive during these hours. On the other hand, if you spend your time watching youtube, stalking people on social networks, reading reddit all day long - you are in the environment of your work, but not actually doing it. It's not a conscious decision, but it does happen due to comprehensible reasons.
I'm almost working, but... I mean I'm not doing things that I would do if I was finished with work
This is my most common problem. If you are not conciously skipping work then you are procrastinating. I could trace this back to the high school years. When you have undone homework you always have that guilty feeling somewhere and can't enjoy yourself so freely. On the other hand, once your task is complete you can do whatever you want, without feeling guilty about it. The same thing happens in your current situation. So what can be done about it?
Most often, your task is too vaguely defined. For example, "Resolve global warming". If you were given this task you would not be able to work on it productively as you'd have no idea what to do. If you had a boss watching over you, constantly polling - how does that resolving go hmm? - you would be nervously sitting around, but basically doing nothing. Not doing your work (because you can't), not improving your health and not pursuing love. The reason behind it is that your task is not defined well enough and is not split into microscopic subtasks. Even if you think your task is straightforward, it is not. Take some time to imagine working, the steps - without actually doing them. You can do this with your eyes closed, laying on the bed, without lifting a finger. Imagine that steps in your work are getting done in seconds instead of days. Think of their result, and what would you do after that is complete. Most often, when I'm done with a task I start procrastinating again because I don't know what to do next. Not because it's not on the to-do, but because it's not defined explicitly. The take home message is this: write a very detailed to-do list from which you can tick off subtasks every hour. It'll feel like you are actually doing something and it'll help with progressing.
Lack of motivation
Get yourself something to anticipate. It's important to have at least a small treat everyday. Most importantly, weekends are there for a reason. Even if you usually work the weekends, come up with at least one day per week when you are not doing anything even remotely connected to your work. Binge watch your favorite show, go outside or on a date, spend a day in the bed reading or whatever. Think of that day during the weekends and imagine that it's a reward for your hard work during the week. If money is something that motivates you, change it for something that you buy with that money. It's much more exact as something to anticipate. It sounds better to await a travel to Spain that is life experience, several days off work, sunshine and beaches ...than $5,000 which is just a number on your account. Both are of the result of the same work, but what you'd spend the money on is more rewarding than just a transfer between banks.
In essence, if you have nothing to work for, god forbid you think you have it all figured out and have all the money in the world, then you are actually in deep shit. That's why I don't want to win the lottery. It is exactly the same as entering a cheat code in a game. The fun gets lost, there is nothing to play for. Nothing to live for. It's not a problem if life is not so easy and comes with struggles every now and then - provided you have every basic need fulfilled and also have fun as the reward.
And lat but not least, the most selfish and arrogant bit you can get away with... Think of a scenario that is very bad and be happy about that fact that you are not in it. If this won't get you working then nothing will. Think of the misery of billions of people who are less fortunate than you, who work their asses out just to have some food on their table. Most people in rich countries don't have any problem with food and create fake obstacles to justify being lazy. This is kind of the first world problem effect. Are you procrastinating because of blatant reasons, while so many people would happily switch with you to solve your problems. The mere fact that you are able to procrastinate often indicates that you won't starve or get fired by doing so, it's just an inconvenience to admit you are not that productive. Many workers around the worlds don't have this luxury.