What are you feeding them? Cats can have food allergies.
My dog is an example. The vet recommended a puppy food when I got my Golden Retriever. Poor pup! His hair fell out, and as the puppy food had ground corn in it and Goldens are, in general, allergic to corn, I fed it to the birds. So I started feeding him lamb and rice mix. He is now 5-years old, 80-pounds and all play!
Cats, unlike dogs who are generally tolerant of tubers and rice, cannot digest grains. You can feed them the following: finely ground chicken, fish, or rabbit, mixed with eggs, yogurt and human-grade calcium. You can make this in batches and freeze it, as it freezes well. Best of all, if you have a fishing license and raise your own chickens and meat rabbits, this is a good way to save money and improve you cat’s health.
Two things you will immediately notice is that their litter boxes will no longer stink of ammonia and their feces are very small. Your cats will not eat much because they will feel full sooner than they would eating kibble.
Oops, I see you beat me to it.