I've experienced similar charging issues. I'm using the mophie stream pad plus (10w advertised) qi charger. When I placed the airpods on the center of the pad the mophie LED would blink rapidly indicating that there's a foreign object on the pad, thus not charging.
Experimenting a bit, I placed it above center just below the connecting usb cable to qi pad. The mophie LED stayed on for a couple minutes then turned off but when I tapped the APP case, it lit up amber/orange...indicating it's charging still? I'll find out soon enough. I left it on the pad close to midnight in this state and will check back in an hour or so. It was at 42% before I placed it on.
EDIT:
Just took it off the charger. Now showing 81%. Guess it's all about placement?