Stress, worms, lactose intolerance. I have one cat that looked like he was going to have to be on kitty Zoloft for life, but magically turned around with some love and time.
I hate to say it, but the more expensive food is better for the cats. You don't have to buy the vet food though -- there are more moderately priced items at PetSmart or a local feed store. A loose rule is that corn should not be in the first three ingredients.
Signed,
Official cat lady