I haven't read all the responses, but my idea is a different one from the others I read.
My neighbor had a dog, I think a lab, that did this so much he got open wounds. Same areas over and over. Back to the vet again and again with no success. Would get it cleared up, use a collar to keep the dog from getting to it, but when the collar came off, the dog was back at it. Eventually the vet decided the dog might have a 'nervous condition' and they put the dog on an anti-depressant. Both my neighor and I howled about this, but she was willing to give it a try, and I couldn't believe it but it worked. I guess the dog had some kind of OCD about this scratching thing. After about a year on the meds, they weaned him off and he was fine and didn't go back to his old habits.