Does the receipt show when you attempt to enter the vendor's invoice in AP-C?  
 
If not, if the goods are here, and there are no items waiting in RNI/AP-C, a quick fix is to enter the invoice in A/P-B, so the vendor can be paid.  Sometimes we are missing a receipt, usually the result of a record lock when receiving purchase orders, and I compensate for it by using AP-B and do not charge the GL for inventory, but the GL for "Received/Not Invoiced Liability" Account.

Quote from fast on 10/30/07 at 13:40:14:

 
Are the PO receipts going to Inventory or to QC Inspection?  If they are going to QC inspection, the receipts will not show in AP-C to match to vendor invoice.  QC Inspection receipts need to be bought off in PO-J-C, Inspection Buyoff.   Occassionally, our receiving person slips a finger and a receipt will end up in QC inspection by mistake.

Go to AP-C as if to enter the invoice.  When you get the message that there are no items available to invoice, click "View All"  and check the detail.  Was the PO Closed?  Processed with a different invoice?