Right front speaker for AirPod series 2 not working

As I said one of the speakers on my right AirPod has stopped working. They are series 2 and about a year or so old, but in good condition. The only thing is the case got a little dirty, but the airpods are fine. I have tried un-pairing and re-pairing them to my phone and it doesn’t work. If you have any suggestions please tell me. Thank you