Try frozen bloodworms, if they won't eat those it's hopeless. I once had a large half black veil angel, then added some small tetras to the tank. The angel started eating them, so I isolated it in a net breeder thinking to give the tetras some time to grow larger. The angel refused to eat, even after it was released from the net, and starved to death. I was watching it one day, and saw the color start fading rapidly (less than one minute) from the edge of the caudal fin. When the color loss reached the actual tail the angel died.