Did an account called Credit Card Receivable mysteriously appear on your Balance sheet in QBO?
What the heck?
A credit card that you or your client tried to process in their file through QB Payments was declined.
You should also have seen an Alert like this when you login to the QBO Database.
The sales invoice, or sales receipt is still marked as paid, so it is kind of deceiving. But QBO will have automatically created an account called Credit Card Receivables, which is holding those balances you've not yet received.
Click on the blue Payment Processing Errors link and you will be shown a table of what cards were declined and why. You also have the option to retry the card directly from that screen.