Left side air buds won’t work

Try a full repair, reset, and try again.

Settings > Bluetooth > Airpods Pro > Forget this device. Confirm it.

Put both Airpods in their case, charge them via Lightning for a few minutes (or preferably until the case is 100%). Unplug them. Open the case and hold the button on the back of the case for 15+ seconds or until the light flashes white. Hold them near your iPhone and see if the pairing screen comes up and pair them. If not, repeat until it does. I don't want to advise you to manually pair them because I heard others with a similar issue when they paired them without using the built-in pairing feature. But if you can't see the screen pop up, then manually pair within Bluetooth.

If it doesn't work, open a case with Apple Support here: https://support.apple.com/airpods