Your AirPods case must be a wireless charging case (Qi) to charge on the dock. Some AirPods, such as second-generation AirPods, were sold with either a standard case or a Wireless Charging Case.

To tell them apart, look at the status light:

• Wireless charging case: the LED is on the front outside of the case.
• Standard case: the LED is inside the case, visible only when you open the lid. This case does not support wireless charging and must be charged with a cable.