if your cats go outside also, are they on Frontline or something to prevent a flea from getting on them? a flea bite can set off a chain reaction if your cat is allergic to them.
Solid Gold Health makes a product, it's dehydrated sea weed, we used to get it for our old lab, she was 15 yrs when we lost her. But it helped her dry coat and arthritis. Worth looking into.
Might be a food allergy too, lots of cats are allergic to chicken, one of the most popular ingredients in cat food, might try switching to Purina One, for sensitive systems, no chicken in it.
Keep us posted...