Demonstration of the Problem
I have two, complete sets of Lark transmitters and receivers. The transmitters and receivers appear to be fine. The cases are not; they have different issues. In one case, the left charging slot seems to be dead. In the second case, the receiver slot is problematic. It drains a charged receiver, and it fails to charge a depleted receiver.
(In my video recording, I may have confused the transmitter-receiver combinations. Could the lack of pairing may have been because the units were already paired?)