Slight variation on the answers to date, however my AirPods Pro had one headphone drain to 0% whilst the other would still have 50% despite always using and charging them together.
I found that wiping down the inside of the AirPods charging case with a dry cloth tissue solved the issue. Turns out that if there is even a hint of a foreign substance (Eg. Crumbs, fluff, wax, liquid) on the inside of the case where the AirPods are charging, this may prevent one or both from charging adequately, if at all.
I wiped the inner case, put the Pods back in, and they both charged again and return to equal battery use. This was after resetting the case, disconnecting and reconnecting the AirPods, using the “forget this device” method and anything else I could find. If that proves useful, then great!