Inventory Business Units and PO's

We are having an issue with creating replenishment requisitions for inventory. The replenishment requisition contains the inventory business unit, and when it is sourced to a PO, the inventory business unit appears on the PO; however, at some point during the PO being worked by the buyer, the inventory business unit is then blank. When the inventory item is received using the receiving pages, the receiving processes will not 'move' the item into inventory and update the quantity on hand. The users all have the correct permissions to receive (interface user).

Because this is not working, our inventory receiving personnel must manually receive the item into inventory, which is time-consuming.