
The thing is, earmites in dogs is really quite rare. It is far more commonly seen in cats. I remember when I told the owner of the queen who had infected my stud cat (who then infected one of my own queens, who then infected everyone else!) that her queen had earmites. She was surprised because "I have been treating her with Thornit for weeks already!"
The gunk in the ear needs to checked under a microscope to confirm ear mites.