Thanks for getting back to us, sim-pumbing.

I'd recommend checking the invoice if there are no credits applied to them or if a previous balance was forwarded. You can turn on the Account summary on the Custom form styles to see if there's any Balance forward amount shown or not. Furthermore, you can also run a Customer statement to see customers' history. here's how:

1. Go to Bookkeeping, select Transactions, then select All Sales tab.
2. Select the Filter ▼ dropdown.
3. In the Type dropdown, select Statements.
4. Select the date range from the Date dropdown.
5. Select the customers you want to see statements for from the Customer dropdown.
6. Select Apply.

If in the event no balance forwarded is shown or no credits are applied to the invoice. I would recommend reaching out to Customer Care Team. That way, they can further check the invoice and customer's transactions in a secure form. 

1. In your QuickBooks Online, click the Help menu in the upper-right corner.
2. Select Contact us and enter the details of your concern.
3. Click Let's talk and choose Chat or Get a callback.
4. Enter the required information.

Feel free to reply to this post if you have more questions.