Here's what could be causing your AirPods to not charge as planned:
be sure your charger supports QI Wireless charging. Case-Mate chargers support this technology.
How to determine if your AirPod case will work on a wireless charger:
The 1st Generation of Airpods are NOT Qi-enabled and not all charging cases will be indeed a wireless charging compatible case. You may or may not be able to use a wireless charger depending on the case you have!
The cases look similar but one will have the green status indicator light on the front- (if it is wireless compatible). A regular lightning charging case will not have the status indicator light on the front- it is on the inside of the lid only. You can also lift the lid of the charging case and see what model number charging case you have. A1938 is wireless charging compatible, A1602 is not. You can see the difference in the images below.
To verify that the AirPods charging case you have is indeed a wireless charging compatible case, the article below from Apple will also help you identify what you have.
We do have a workaround for you. You are able to use the charger, however, the AirPods case needs to be hard-wired into the Wireless charger. There is an additional USB port in the wireless charger that you can plug your regular Airpods case into to charge.