I also had the same problem, one airpod not working despite the fact that both were charged in the case to the maximum. Making it work was assisted by the act of cleaning the charging contacts on the inside of the case and the AirPods themselves since dust can prevent charging. I then restarted the AirPods through pressing the set button on the back of the case about 15 seconds until the light turned amber. Afterwards I recharged them with my phone. Make sure to also adjust the audio balance in the phone settings since at times it is shifted to one side.