@sercrojoe said:
Edit: By changing certain groups' or components' Entity Info, I've been able to change their layers (apparently), but for some of them it just won't work. Why is that?
Well, that's the easiest way and actually "the" way normally. You can also use the Layers toolbar but the results will probably be the same.
If you have any issues ("some of them not working..."), double check if the content is on some different layer than Layer0. "Primitive" or "raw" (i.e. ungrouped) geometry should never go elsewhere. There may still be nested groups/components on all different layers within your groups/components - check that, too.
Eventually copy the troublesome groups and paste them in a new file. Have a look what layers they have "inherited" to get some clue what may be wrong with the original file and/or ultimately upload such faulty pieces here so we can have a look.