These settings are not compatible with each other. Make sure you don't accidentally use both. Load more will take preference over Max rows and also the Hidden limit setting. Assuming that you used Max rows intentionally, there is a better way to achieve a perfect justified block without an orphan row. Go to the General tab > Row behavior section > Incomplete last row setting and choose the Flexible option. Play with the Load more limit setting to achieve the number of images you are comfortable with initially and also in one go.
When Load more doesn't show up, albeit you set it up correctly... The gallery needs to have enough content first, that is split into smaller chunks by Load more. Make sure the General tab > Thumbnail count and dimensions > Limit image count is set according to your needs. Especially when you are using external sources or WordPress Recent posts - see the fix for: I only get about 25 pictures. Also, it could be that you just don't have enough content to use Load more with. For example, RSS feeds can be limited in this regard.