My initial thought was "protein itch." If the food is too high in protein, the excess makes them itch.

I tried grain free foods, but those tend to be very high in protein. After lots of trial and error, I settled on Halo Spot's Stew CHICKEN. (I have one girl who can't eat fish, so the salmon was out.) Halo uses barley and oatmeal, two rarely used grains in pet foods, so they haven't had time to develop allergies to it. So that is one thought.

I looked up the formula you are using; low protein, and nothing jumps out and says "problem." Of course, the Blue Buffalo line in general is a high quality food.

What do you use for treats? Does anyone else besides you feed the dog? (family members, day care) Could that be the source of this issue? Just a thought.