This is a firmware issue. After extensive testing I have come to this conclusion. At first I thought only the right microphone was working on my AirPods. However I got my left mic working under certain circumstances, so I know that it can’t be hardware failure. Apple please sort this out it’s very annoying. I think it started when you introduced split gesture controlling, to enable us to pick a different double tap result for each pod, because up until that point I didn’t have any problems with people not being able to hear me. I have called my landline answerphone repeatedly using different settings. The answerphone has the advantage of disconnecting after a five second period of silence so I know exactly when each mic was operating and when not. Interestingly I could get my left mic working (an audible message was played back on my answerphone) when only using that pod (with the right pod left in the charging case).
I guess the short term answer is to find out which mic is working for you and set and fix it to that mic in settings / Bluetooth/Whoever’s AirPods/ i
Long term Apple please give us a new firmware update. Thanks.