Try one of the LID Natural Balance formulas. Go with a novel protein like fish, venison, or buffalo for example. Keep her on it for a few weeks and look for improvement.
Add extras like coconut oil, salmon oil, Olewo carrots/beets, yogurt, probiotics.
What kind of flea/tick prevention are you using? Do you apply lawn treatments? Is she around anything that might trigger an allergic reaction? Allergies and skin issues are common problems in the GSD - hope you find a solution