Hey, have you tried live foods? That worked well for my old betta and my angelfish. Cause like, angelfish are predatory kinds of fish, maybe if you toss in some really small baby fish or insect lavae, they might starting eating for you. Most fish like food thats moving.
However, if the live food doesn't work, they might just be over crowded, so you should probably move them into another tank. Even if they aren't, move some of them anyawy, in case they're just stressed out because of each other.
Test the waters pH as well and adjust it if it needs it. Other than that, I'd say maybe they've got an illness of some sort. I'll do some research and see what else can be done for them.