The most likely scenario is that you've chosen a preset in the shortcode, just like this:
[$justified_image_grid preset=2]
Presets simply carry a number of settings, and override global ones. According to preset authority, you can influence which settings to pass through presets, or include with presets. Furthermore, any setting that you added to the shortcode will override its global variant. This allows galleries to be styled individually. So if you get no change after playing with a global setting, check if you have a preset or that very setting in the shortcode or not.
Another likely case is that you use a caching plugin. Changing the global settings of JIG does not invalidate the cache on individual pages and posts. The caching plugin does not know where the changes should occur, as it does not know about JIG. Just purge the cache after changing global settings of JIG.
Please check Custom presets and authority explanation and the Mobile specific settings have no effect fix too.