Allergies are so difficult to diagnose. She could be itching from food, fleas, forced air heat that makes her dry, or plant allergens.
Given that it is winter, hopefully you can rule out plants and pollen. Have you looked at switching to one of Natural Balance foods? Some of them don't have grain and they tend to have single, uncommon proteins like venison and duck. Feeding this for a couple of weeks would tell you if her problem lies with grains and/or proteins like chicken and beef.
She may also be dry from indoor winter heating. Have you tried giving her fish oil as a supplement? I've heard that people have good luck with this.
Given that it is winter, hopefully you can rule out plants and pollen. Have you looked at switching to one of Natural Balance foods? Some of them don't have grain and they tend to have single, uncommon proteins like venison and duck. Feeding this for a couple of weeks would tell you if her problem lies with grains and/or proteins like chicken and beef.
She may also be dry from indoor winter heating. Have you tried giving her fish oil as a supplement? I've heard that people have good luck with this.