If your left or right AirPod isn't working (Multiple Devices)
If you own multiple apple devices which you usually connect to your AirPods, don't forget to remove your AirPods from all these devices too before you do this solution Apple has suggested.
The solution didn't work for me last night until I finally realised this morning that AirPods immediately connects to a nearby device it used to attach when you remove it from one device.