I didn't have this issue personally. I have AirPods 2nd gen, and I think the source of problem is that the magnets are damaged. The magnets are used to charge the AirPods while they're in the case, the cause of this problem might have been because of Falling, etc. Try putting the AirPods in the case and turn the case upside down, if they fall, it probably means that the magnets are damaged.
Here's a solution I've found online, I hope it fixes the problem:
Reset your AirPods Case:
Resetting your AirPods case is going to clear the operating system which could fix the charging process. To reset the AirPods case:
Press and hold the setup button on the back of your case. Let go of the button when the status light flashes amber, then flashes white.