Hormone issues causing hair loss are quite common and because of the way hair grows in dogs (excepting a few breeds) it can take many months on medication to demonstrate any improvement.
Basically, the hair in dogs doesn't constantly grow like our hair does. There is a long inactive phase where the hair is just there and attached before the next phase where it is shed and another hair starts to grow.
As long as there are not other symptoms like itching, scabs and sores, etc., don't expect too much too soon. If there are other symptoms, revisit your vet as there are so many possible causes that we can't do an adequate job diagnosing here.
Doug, DVM