Multicube query issue

Hi guys, i have a problem with the result of the query being run on a MultiProvider. Multicube is based on 4 cubes, COPA CCA I/O Planning Cube, Cost Centers/Order Commintments, Cost Centers/Orders Actuals, and the latest that was added is Purchasing Data. The query has contains Order and PO # (along with a few filters and a couple of restricted keyfigures). But each time i run the query with different variable entries, the PO #'s come as unassigned. What could be the problem? How can i diagnose it ?