It is pretty easy to confuse several other diseases with mite infestation, including atopy,food allergies, ringworm, bacterial skin infections and flea allergy dermatitis. If the mites clear up but the problem doesn't it would be a good idea to consider the possibility that there is a secondary infection. Persistent mite infestation may be associated with FLV or FIV. If treatment isn't successful it may be worthwhile to check into this.
The tresaderm is used for bacterial infections, and I am assuming that you took your cat to the vet because of a skin infection, hair loss or you saw the mites. At any rate, I would get a 2nd opinion if you are not happy with your vet, or if your vet refuses to treat the problem to your satisfaction.
I hope this answers your question!