My youngster was losing his fur in patches last winter, I was convinced it was food allergies. My vet said it was likely a contact allergy. Sure enough,homemade organic food didn't help. Since I don't use any chemical household products (no cleaners, rug or room deoderizers, scented candles etc) I was mystified.
You need to become a detective to find the problem. In Sunny's case, the problem was a dust allergy: I moved his crate close to a heat vent so he wouldn't be cold! As soon as I moved his crate away from the vent his fur started to grow back. I moved to a house with wood heat this summer and he's been fine this winter.