for all the common angels feeding then dying without reason,
the main problem usually lies with flukes.
fluke infestation is common but easy to cure.
freshwater dip removes all adults on the body. a little flukes is fine but most wild caught dwarf angels are riddled with flukes.
flame and lemonpeels are prime suspects.
try it the next time you buy a wild caught flame angel. dip it in freshwater for a few minutes. you'll be surprised what you find dropping off it's body.