I have started having the same issue consistently. I receive some items against a PO and receive a bill for those items and then I am unable to receive the remainder against the PO because it is not available in the list to select for item receipts or bills. When I look at the PO, however, the additional items are still showing not received and not billed. Is this an issue with PO's created prior to an update? for me it would be update /release 22.