I rec'd emails from QB showing an invoice was paid, but after 3 weeks, it is still showing overdue in my list of invoices. How do I determine the cause of the error?

If you're using QuickBooks Payments, the invoice will mark itself as paid automatically. But if you're using a different payment service, you'll need to mark it manually. 

Go to Invoices > Find the invoice > Click the small arrow icon in the Action column. Then, choose Mark as paid.

Let us know if you have further concerns.