I use a vinegar rinse as a final rinse when bathing my dogs. It cuts the soap film and returns the skin back to the proper ph. If I do not do this, she digs and digs and digs.
My Airedale was very itchy when she first arrived (as a rescue). However, flax oil and Vitamin E caps taken orally have made a huge difference for her. No more constant chewing! It also makes her coat gorgeous. You can also try Omega-3 caps instead of flax oil. Pet stores often carry these, or you can find them through pet catalogs. If that doesn't help, maybe get her tested for allergies. It's also possible she's allergic to fleas. Even on a preventative med, the fleas have to bite before dying, and if she's allergic, she'll react to the bite.
I had this with the Dachund I had, they gain a lot of weight on Prednisone ,and he drank copious amounts of water, I may have been unlucky but he got diabetes and the Vet said the prednisone certainly hadnt helped,have you ever thought of googling in case there is any Natural remedy out there, I will also check around I go to a great Health food store they may have Animal stuff. .