Hi--
There are a lot of possibilities then...
1. Have you gone to Transactions>>Purchasing>>Edit Purchase Orders, picked the PO, then picked the line, then clicked on the quantity status icon (the blue and white icon) to see the status? As it sounds like the amount has already been invoiced.
2. Have you gone to Inquiry>>Purchasing>>Purchase Order Documents and selected to view both purchase orders and receipts to see if there was a Shipment/Invoice or separate Shipment and Invoice recorded for the item before?
3. To confirm the shipment was posted? As you cannot invoice until a receipt has been posted for the item.
4. If you cannot find any documents suggesting that this was previously invoiced, then you could try running the purchasing reconcile available under Tools>>Utilities.
Take care,
Christina