Is the allergic dog on Frontline or Advantage? If not just one stray flea bite per month can cause the dog to get hyper-allergenic to other allergens like inhalant allergies and food allergies. Try using either flea product for 1 month and see if the other allergies end and hair grows back. that often is the most helpful and least hurtful [compared to steriods] you can do. When those two products first came out we[at a veterinary hospital i worked at] put many dogs on it who had no hair on their back and tail region for YEARS the hair grew back within 2 months of starting consistent use of Frontline or Advantage.
Also it is important for the omega 6 and omega 3 fatty acids to be in the right balance if you are trying to use it to combat allergies. If there is too much of omega 3 it can make the itching worse