Some medications can take up to a month to start working. I recommend bathing her in a soothing shampoo once a week to remove environmental allergens, and wiping her down with a antibacterial baby wipe every day when she comes in from walks. For a shampoo, try ones with soothing agents -- lavender, aloe, callandula, wheat grass, etc.

It could perhaps be a food allergy. What kind of food is she on?