My 8 year old cocker spaniel was sleeping and jumped up and started scratching his left ear and shaking his head. I thought something crawled in his ear the way he jumped. I took him to the vet and she said his ears are fine. She swabbed his ears and looked under a microscope and said there were no signs of any infection or mites or anything. His ears were not red or swollen. There was no wax or any signs of an infection. She flushed his ears and told me to bring him back in a week. I brought him back and she couldn't find anything wrong , but he was still shaking his head and scratching his ear. She said the only thing she could see was a tuft of hair right in front of his eardrum, but said that many dogs have that and she didn't think that was the problem.
She gave me Enrofloxacin 136mg and told me to give him one a day for two weeks in case he had an inner ear infection behind the eardrum that she couldn't see. After giving him the medication he is still shaking his head and scratching his ear. He also is not hearing as well. The vet can't find anything wrong. Does anyone know what the problem could be?

If your vet is out of ideas, perhaps it is time to try another. No vet is going to find everything on first try. It is quite obvious he has a problem. She is not finding it. Since different things often produce similar symptoms, what somebody else's dog had, might not be the same thing.

HI,
I agree with the answer before mine, please see another Vet. There is a reason, as you know, why your dog is shaking, scratching at, or paying attention to his ear. The other Vet might take a blood sample, have it analyzed, and might possibly know what is wrong, or what to treat, from the analyses.
Notice the word "possibly" or "might" take a blood sample. That is up to the Vet. I am not a Vet, but a dog lover and owner. I do wish you, and he, the very best.