Wax especially if you use Q-tips quite a bit...
If you have Hydrogen Peroxide, I suggest taking a cap full and lie on your side, and pour a cap full in your ear, to allow it to penetrate pretty deep. This will help with that problem.
Sometimes I get ringing in my ears but my left ear isn't as well hearing as my other one, I might try tmyelf.
Might save you a doctor's visit.