Let's pull up and review the details of your unpaid customer's invoices so you can get back to working order in no time.
First off, you may have received the invoice payment from your customer yet the payment wasn't linked to the corresponding invoices. If this is the case, you may need to record your customer's invoice payments. Here's how:
Go to the Sales page.
Find and double click to open your unpaid invoices.
Click Receive payment.
Enter the amount you received from your customer.
Select Save and close.
To add, you can read this article to learn more about how you can record a customer's invoice payment: Record a Customer Payment.
Meanwhile, if you're receiving payments from your customers directly from their invoice, you'll have to match the downloaded payments to the corresponding invoices.
Lastly, you can always visit our Help Articles for QuickBooks Online if in case you need some tips and related articles for future reference.
If there's anything else that I can help you with, please let me know in the comment section down below. I'll be always around ready to help.