Unable to Match Invoice to PO

I have a case where I've entered a PO number into the invoice header and the supplier and site information populated normally as expected. However, when I click the "match" button in the invoice workbench it does not find the PO. I changed the match action in the invoice header from PO to Receipt and experienced the same issue. We received one of the PO lines and clicked the match button again, this time it brought up the receipt for matching but when we tried switching the match action back to PO it reverted to not showing anything when I