We have experienced this problem before and it is very confusing.  After entering all the appropriate info for the PO in AP-C, we are getting the following error message: "There are no Receivers found for this vendor before 02/06/06. Would you like to view all receivers?"
Of course the material has been received, but the system does not recognize it.  Any suggestions?

A closed PO will cause this also.  If you look in PO-D view PO receivers and see an entry for the PO with a C next to it it means it has been closed ( we have had this happen randomly and the person receiving doesn't remember closing it except for receiving purposes) and if you re-open the PO you will see the received item in AP-C.