Please follow the vet advise as skin problem can be caused due to several reasons. So some times it take long to find them and treat them. Their can be several cause of skin problem such as bacterial infection, Fungal infection or mix of both, open wound, dry skin and allergy. You vet may perform few tests to find the above causes and will provide the treatment accordingly. As told earlier skin problem may take long time to cure . Take your dog to vet when asked for follow up.