It could be allergies. Prince's scratching got so bad that I finally had allergy tests done on him a few months ago. He is not only allergic to a lot of foods (beef, buffalo, carrots, peanuts, pecans, eggs, etc.), but he is also allergic to all kinds of grasses, weeds, trees and well, just about everything inside and outside the house
Changing his diet helped a lot. Now that the weather is nice and he is spending more time outside, the scratching is getting worse again. I'm really trying to avoid having to give him shots.