This could be a few things: food, or health issues. Check with your vet to make sure that there is nothing wrong with her health such as allergy or other health-related issues. Then you can look at the food. Most food that contains corn as the 1st ingredient, you should avoid. You can read more about best diet for your dog at:
From Best to Worst - My NEW Rankings of 13 Pet Foods
I found that adding fish oil to my dog food is a good longer term cure. Some people favor coconut oil or olive oil but I sware by good fish oil. You can also improve the condition of Macy's skin by using choosing a good shampoo. My favorite one is Isle of Dogs - Silky Oatmeal Shampoo. A leave-in condition also helps temporarily.