Hello pickledkid,
To help rule out a hardware issue with your left AirPods, first make sure the contact in the case that charges the AirPod is free of dirt or debris so that it is making clean contact with the AirPod.
Further tips are discussed here:
If your AirPods won’t charge - Apple Support
If that doesn't seem to be the issue, resetting your AirPods will clear out any minor software issue between them and the iPhone: How to reset your AirPods and AirPods Pro – Apple Support
- "Put your AirPods in their charging case and close the lid.
- Wait 30 seconds.
- Open the lid of your charging case, and put your AirPods in your ears.
- Go to Settings > Bluetooth. Or go to Settings > [your AirPods].
- If your AirPods appear there as connected, tap the More Info button next to your AirPods, tap Forget This Device, then tap again to confirm.
- If your AirPods don't appear there, continue to the next step.
5. Put your AirPods in their charging case, and keep the lid open.
6. Press and hold the setup button on the back of the case for about 15 seconds, until the status light on the front of the case flashes amber, then white.*
7. Reconnect your AirPods: With your AirPods in their charging case and the lid open, place your AirPods close to your iPhone or iPad. Follow the steps on your device's screen."
If you still have questions, reach out to Apple Support directly, as this may require closer guidance: Get Support
Cheers!