pH sounds a bit low . I would bring it up to 8.2 - nitrites should be 0 . Are you doing partial water changes?
My clowns love live brine shrimp - you can buy the eggs at pet stores and hatch a batch within 24 hours. I hatch them in one of the small critter cages ( like the type used for hermit crabs).
I have noticed that my clowns will get very excited for live food. Perhaps it might stimulate the appetites of your guys.
Also - are they tank bred or tankraised clowns? Some of the wild ones are poisoned in order to capture them. The long term effects are very much what you are describing. I have had great success with the tank raised and bred.
Best of luck