It can take a while for a food allergy to clear a dog's system, sometimes well over a month. And the food allergy could be to the protein source not the grain. With allergies it can be a hit miss nightmare finding the culprit. That being said it could also be an enviromental allergy, or it could be both. I know that can be a lot to take in. I had a dog with both food and enviromental. My house cleaning needed to be steped up. Vacuuminmg and dusting sometimes twice a week. Steaming my caprets monthly, except in late fall, when I did it at least every 2 weeks or weekly depending on how bad things were that particular year. Air filter must be changed at least monthly. Our laundry detergent is mild, (but that has always been true because I have allergies to detergents.) and I didn't use fabric softners on her bedding. I hope you find the cause soon, and it is something easily worked around.