Is it possible she has a flea allergy? I had a dog once who never had a flea on him, but would literally drive himself, and us, crazy with his itching, scratching and biting. The vet finally worked out that it was a flea allergy. All it takes is one flea to jump on, take a chomp, and jump off again, and that flares the allergy up.
I can't remember now what the treatment was, because it's years ago and I was just a teenager, but I think he used to get cortisone injection every few months, but I'm sure there are other and better treatments now.