You can scrape repeatedly and miss mange. I'd treat for mange and see if that helped. I had two dogs that were diagnosed with allergies (the relatively expensive blood test indicated all sorts of things). I put them on shots (and it got so they knew when I was coming with the shots and didn't much like it) and they did little good. Finally the vet thought we should treat them for mange because a visitor's dog came down with mange.... Bingo. However my dogs had gotten very very very bad with mange in the time we were treating for allergies. My wallet had gotten significantly lighter, too.
So I'd be tempted to treat for mange.