Why is my cat still shaking his head after ear infection drops?

Ear infections are relatively common in both cats and dogs. Some are more persistent than others. As you said, your vet took a look at the contents of the ear to determine what course of treatment might be required. Without a true bacterial / fungal culture, I could only speculate as to what the exact organism might be. The majority of ear medications, tresaderm included, are traditionally quite broad spectrum in an attempt to cover as many bugs as possible. The reason Fatboy is shaking his head is likely a combination of both the drops and the discomfort associated with the infection. If after six days of effective treatment, you are not seeing any improvement at all, you might want to call you vet and let him know. He may want to obtain a new sample to see if any progress is being made. If not, a true culture could be sent out to better target his therapy and change medications if needed be.