I have a 10 year old A20 and last summer at Oshkosh the fine folks at Bose replaced my mic and the whole assembly. Today the mic did not work at all in my airplane. Audio and noise cancelling works fine. It worked fine last week! There is no visible damage to the headset or wiring. I tried it on all four of the airplane’s mic jacks and then I got my Bose X out that worked just fine. Weird.
That indeed sounds weird! Especially since your audio and noise-cancelation is still active and working.
You've mentioned trying it in various jacks, with no improvement. Also, as I would usually recommend, you've managed to find a different headset (Bose X) that works fine. That'd leave me to believe there is likely a minor issue with the A20 headset microphone.
However, we are able to help you with that! My advice would be to contact the Aviation team for your region to discuss the service options available to you. To do this, simply head to Global Contact Us > Select your country > Scroll to the bottom of the page > Click 'Contact Us'.
They will find you the resolution you need. In the meantime, let me know if you have any other questions!