what causes one of the Air Pod Pros not to fully charge?

The last thing you can try is to clean the charging port on the case.

I assume that you've consulted the related article at the top of this page: If your AirPods won’t charge - Apple Support. If so, you may have a defective case as it would explain why neither AirPod charges. Your best avenue would be to contact Apple Support to see what they think: Get Support.

Make sure that there are no debris in the case and that the metallic stem end is clean.

I cleaned the case and the metallic stem. Still not charging. How clean does it have to be?! I keep having this issue with either one of them.