I also have had this problem. with my dog it turned out to be a allergic reation to fleas. it was very frustrating because I tried everything and nothing would work and the vets were not comming up with anything. She was ultra sensitive to fleas. she wouldnt have bites all over her so we were confused. I used advantage frontline and some others but it was an on going battle. I ended up just having to make sure there was nothing like bark dust (wich fleas love as a breeding ground unless its cedar) or anything around that would attract or promote fleas. And spraying the yard, house ect. We also added garlic to her diet. My dad said fleas will stay away if you eat garlic. dont know if its true but it was worth a try!