Have you had an allergy panel run on your dog? If so you should be able to isolate exactly what is causing the allergic reactions and avoid them. Also, I would highly recommend feeding a food that is "hypo-allergenic" if you are not already doing so. A topic Dermatitis can mean having an allergy to anything from pinestraw to the detergent you wash her bedding in.
I got a Lil feedback from my vet (she a little on the holistic side) She says that a lot of dogs have a natural occurring allergy to soy meal. When I started looking at dog food labels, a lot are holding.. When I changed "splotch" over to lamb and rice, BAM! No more itchy scratchy show! It's not that much more expensive, and a small price to pay for your "kids" comfort. You might try a Lil peroxide in with the shampoo for the next washday. It also helped out a great deal. Good luck.