Try switching to a food with a different protein source, or with a different grain content than you've been feeding. Test this food for 6 weeks and see if there's any difference. If food does seem to be the problem, rotate different foods through your dog's diet. Allergies develop after exposure to an ingredient, and the more exposure, the more likely an allergy will develop. Another food-related cause of itching may be mold.
Molds grow on wheat, corn, and peanut hulls used in petfood.
These produce toxic by-products called mycotoxins, which can suppress the immune system, leading to itching. If you live somewhere humid, or if your kitchen and bathroom are unventilated, mold spores may be causing your dog's itching. Mold grows wherever and whenever there is moisture. Depending on where you live and what your drainage conditions are like, this can be seasonal or year-round. The itching follows the mold growth.