try and get a good mix of the following into the tank
good quality flake, live food (bloodworm, brine shrim etc), frozen bloodworm etc, sinking food (tetra prima is really good), fresh veg (cucumber goes down a treat), dreied tubifex worms, if you've catfish or plecs then sinking catfish pellets and algae wafers. erm that'd probably do it, I try to get a new thing of food each month, whatever takes my fancy so they get loads of variety, different things everyday. They can get a bit constipated if they get too much of the same thing and that can stop them eating.
Ian had a problem with some of his angels stealing all the food, so he got frozen food, dissolved them in some tank water, put some in one end of the tank, most fish went to get them, then he got some up in a syringe thing and squirted it in right infront of the fish who wasn't eating's face. worked a treat and he's coming on well now