[anonymous]

Make an appointment with a veterinary dermatologist, asap, for exam/testing and effective treatment options.
Many allergens are airborne and present all year round (indoors/outdoors)
You are wasting your time changing foods over and over again. Meanwhile the dog is suffering (from the symptoms you describe).

Hope this article helps: http://skeptvet.com/Blog/2010/06/evidence-based-canine-allergy-treatment/

you can use the search engine there to look up allergies and other topics.
Good luck