One of my GWP's started scratching/chewing herself to the degree she almost made her back legs bald - turned out to be the new food supplier I had moved to, went back to the old food and she was back to normal in a few days. Also could be something where you walk them, a local wood to me makes both of mine scratch like mad and they get a rash after they walk there - took me ages to determine it was something in the wood !