Have you read my posting above about the internal infection?
One last point, how was the fish respond to beef heart and bloodworms?
These are the Discus favourite food and the fish breeders often feed them with beef heart.
If it's still doesnt eat beef heart and bloodworms, then definitely it's due to internal infection.
And if you are afraid tht the Angel fish will bully the Discus, just remove the Angel fish for a week and see how the Discus response to food.