Unfortunately, angels are known for their hunger strikes. I've had some bad experiences and now I won't buy one unless it's bold, fat and eating in the LFS tank. IME a shy, hiding angel that won't eat is already on the way out. Some that pick at sand/rockwork can be convinced to take prepared foods, but not always. Feeding a fresh mussel in the shell can work really well - angels aren't used to taking food out of the water column, they'd rather pick at something stationary. Some people find that once they're taking mussels, they'll eat nearly anything out of the mussel shell. From there, it's all downhill.