I'm nodding at what Caraline said. The rump is the best place in a flea's mind, and on some dogs one flea bite can cause itching for 2 weeks.
Tibby has a bad problem with naked tail and rear end. We have done steroids, both topical and systemically; allergy testing and injections; medicated baths. The baths helped more than anything, but only for a short time. Diphendrymine (benadryl) doesn't seem to help her, nor does atarax.
But she doesn't do it constantly. She can lay in the chair with Bubba for literally hours and not move a muscle. As soon as they go to bed she starts scratching. And the frightening thing to me is that she sometimes has her head thrown back and has that fearful look on her face that I've seen on some of the SM videos.
Tibby was also on an "allergy diet" (Hills prescription) which didn't help. All of the dogs coats started looking better when I changed them to Eagle Pack.
RIP Wallis, John Robert, Tibby & Pip
Mom to JoJo, Roxie & Linguini (Cavaliers)
and my birds Moji (Senegal) and Walter (Blue Front Amazon)