Singular AirPod not connecting

Try to reset and reconnect your AirPods as shown below:

1. Put your AirPods in their charging case, and close the lid.
2. Wait 30 seconds.
3. Open the lid of your charging case.
4. On your iPhone, iPad, or iPod touch, go to Settings > Bluetooth and tap the "i" icon  next to your AirPods. (If you don't see your AirPods in Settings > Bluetooth, just go to the next step.)
5. Tap Forget This Device, and tap again to confirm.
6. With the lid open, press and hold the setup button on the back of the case for about 15 seconds, until the status light flashes amber, then white.*
7. 
8. Reconnect your AirPods: With the lid open, place your AirPods close to your device. Follow the steps on your device's screen.

Axel F.

Try this, If your left or right AirPod isn't working - Apple Support