Zignature and Nutrisca have no potato, has your vet suggested a prescription/therapeutic diet? That would be the only accurate way to rule out food sensitivities.
Keep in mind your dog may have environmental allergies which tend to wax and wane, they may be mild, seasonal or uncomfortable enough that the expertise of a specialist may be needed.
See my prior posts per the search engine
examples:
How long has this been going on? Because I have a dog with environmental allergies, I had good results, but, only after going to a veterinary dermatologist.
I thought it was the food too, tried all kinds of things with poor results, my dog is doing well now on ASIT allergen specific immunotherapy times 5 years.
Turns out she can eat most foods but does best on Zignature whitefish (before that Nutrisca salmon) as a base.
So, I would consider going to a specialist if her symptoms go on for more than a year (4 seasons) without significant relief.
Food allergies are rare, environmental allergies are usually the culprit.
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For the best results, make an appointment with a veterinary dermatologist.
Here is a good article about what you might be looking at, keep in mind there are even newer treatment options than when this article was written
http://www.nevetdermatology.com/canine-atopic-dermatitis-treatment/
/forums/topic/allergies-and-itchy-dogs/
http://skeptvet.com/Blog/?s=allergies
