I agree with diet. Larger angels need alot of vegetable protein in their diet, as much as 50%, which can be difficult when they come used to eating shrimp etc, they often prefer those types of food. I feed my koran formula 2 pellets, and Ocean nutrition angel formula, and Algae sheets. I don't have a problem getting him his vegetable protien since he wont touch krill, or mysis, which I find unusual. he will just sit and watch the other fish eat until he gets his pellets.
Hope this helps.......... Good Luck