It sounds like the PO is closed if they can't see anything to receive against. Either that or they have PO Receipt Tolerance set and they've received the maximum quantity. If the PO is closed, I would enter a receiving transaction with no PO, and note the reason in the Receiving Transaction. If they find they can't receive without a PO, then check settings in POP setup - there's a checkbox that enables/disables this feature.
If the PO is not closed, they still should be able to receive against the line item in the PO, unless they have PO Receipt Tolerance enabled.
