You charge the airpods in the case and then, about a day later, one of the airpods is totally charged while the other is almost completely uncharged (both airpods having been kept in the charging case the whole time).

I'd had this problem reoccur about once every week or so with airpods I bought last fall. As the problem was mostly happening with the right airpod, Apple originally just replaced that.

After that, things were fine for a while. Then it started happening about once a week with the left airpod.

I finally got Apple to replace the charging case, which I suspect was the problem all along (they also replaced the left airpod).

Hopefully, I won't have this problem anymore as it was super annoying.