Left Outer Join NOT EQUALS hangs.

Question

Complete Newbie to Crystal.  Bear with me.  Dealing with Purchase Orders & Invoices.
Want to report on PO Items RECEIVED, but which we do not have the Bill yet (not Invoiced).  There is a Triple-field key linking both Tables:
PO Receits  Tbl and   AP Invoices Tbl, linking keys are:
PO#, PO Release, and PO Code. 
I want things received, but NOT invoiced.  What I tried was from the PO side, a LEFT OUTER JOIN... NOT Equals.   The system hangs.  Even with a Preview of like.. 5 records. 
Keeping in mind that I know virtually NOTHING about Crystal or SQL for that matter, what OUGHT I  to do within Crystal to produce this report?  (FYI, the tables are Lawson, DB2 under Unix if that helps). 
Thanks for any help!  And it will be good for your karma!  - George -