The amazonian Angel Fish (Pterophyllum scalare) is normally quite a robust fish though not the most active of fishes in a community tank...you should have a slightly acidic water pH and the water should be soft......try making sure you don't have any shells or suchlike in the tank and place some bogwood or peat in the tank to help soften it........the average angel is quite long lived at around 8 years so it could be diet....have you tried feeding bloodworms to you fish rather than just flake food? This can make a huge difference.
For more info try these pages:
http://www.aquaworldnet.org/tas/
http://members.aol.com/AngelBook/