Make sure not to switch foods too often. Relief from food related allergies can take 2-3 months to show up after you switch food. If you aren't waiting at least this long, you aren't giving time for any change to take effect, and are possibly exposing your dog to even more allergens each time. Tracking down food allergies is a trial and error process. It's best to restrict anything but the food you're feeding (no treats/snacks) and wait at least 2 months, if no change, then you can try a different food. Takes patience.

If the itching continues in the meantime or if your dog gets raw areas from scratching/chewing see your vet.