Completed order on POS reverts to pending

We’ve been having an issue for a while now where a team member completes a transaction in the POS and instead of it completing properly, the order reverts to pending in the back end of Wine Direct. This has created all kinds of issues for our end of day reporting and stock control. The difficulty is that it happens intermittently and there doesn’t appear to be any consistency as to why or when it occurs.

Has anybody else had a similar issue or know what might be happening?