Why is my left side of AirPod not charging?

I'm sorry to hear that your left Airpod is not charging.

Take a look for any obstruction in the left side of your case, there could be something, even very small which is preventing your left airpod from charging.

How to clean your AirPods, AirPods Pro, AirPods Max and EarPods – Apple Support

Make sure you are carrying out the charging procedure correctly:

Click on this link to learn how to 'Charge your AirPods and learn about battery life' --> https://support.apple.com/HT207012

If your AirPods won't charge, click here --> https://support.apple.com/HT209506

- If you still can’t charge your AirPods, contact Apple Support --> https://getsupport.apple.com/?caller=sfaq&PGF=PGF32003