we migrated to the online version and noticing that payments that were previously posted in AR are not showing up. Why is this happening?

It's possible that the payment was detached from the invoice, Steve. Let's figure this out together.

When migrating from QuickBooks Desktop (QBDT) to QuickBooks Online (QBO), the connections between invoices and payments are transferred, but they can become disconnected during the migration process.

Let’s take a closer look to see if this is the issue. Once confirmed, we can easily link the payments to the invoices using the Receive Payment feature in QuickBooks.

Also, when running the AR report, ensure that the reporting period is set to All Dates so that we can have a complete view of the data.

You can check this article to know the limitations when converting QBDT to QBO: Learn how terms and data transfer from QuickBooks Desktop to QuickBooks Online.

If you have additional questions about QuickBooks tasks, please add them in the comments, and we will be happy to assist.