I dont know if this will help. But I have two King Charles Cavaliers and I used to feed them Science diet and all the dry food that the breeders recommended. I noticed they hated the food and scratched a lot. Then a friend said why not try raw food as sold in the specialist pet stores. Sounded wrong to me to feed them raw beef lamb buffalo , etc. But I gave it a try and wow the difference is amazing the itching stopped they look great their fur is shinny and they have way more energy.
It seems the pet shop dry and wet food has wheat in it and some dogs like some humans are allergic to wheat "Celiacs"
Anyway I now only feed them raw meat, some fish oil and add a few fresh veggies and the itching is a thing of the past .