I've been fighting the one-dirty-ear syndrome with Denna since the day she came home.
Vet checked them out and said, "just dirt"... no yeast, no mites. The vet really wasn't concerned.
After 5 years of continuous cleaning cycles (still just ONE dirty ear, always the same one), we finally had 1 year of two clean ears... a good round of Zymox gel got them clean, and Snooks Ear Oil kept them clean. Then... we had a relapse this spring. GRRRRRR. I thought it was an allergic reaction to a new supplement that was heavy with flax seeds. I had stopped using the ear oil, so not sure if that would have prevented it or not... but as of this week, 3 months after the first ear returned to it's dirty state, we now have TWO dirty ears. AGHHHHHH!!!!
We're on a round of Zymox gel again trying to get them clean.
She's back on food I know we fed during her "clean period"... so my best guess now is just environmental allergies.
But getting them clean to stay clean has been a major journey for us. I hope yours goes much easier than ours.
If you can nail it down to a food allergy, that would help... but if it's just spring/summer pollen or dust issues, it can be a headache (for both dog and humans!).