Search involving fulfillment status by line item?

Hi all-

We created a transaction column "ship date" as many of our customers will issue one PO with multiple shipments/split shipments for the same goods.  For example, a sales order may have 5000 of X shipping on 4/15, 5000 of X shipping on 4/30, etc, and this would all be on one Sales Order.  As our customers work this way, we had no choice but to create a "ship date" column so that a ship date can be listed on a per line item basis.

Now, I am trying to create a saved search to show overdue shipments.  That is, a search that shows all POs and sales orders that are still unfulfilled and the ship date was before today.  But the problem is, it seems that the fulfillment status is decided for the whole PO/SO, so it is showing me line items that have been complete.  For example, if I have a sales order with shipments on 2/15 and 2/30 but the 2/15 shipment has been fulfilled and billed, it still shows me the 2/15 line item on the report.  I assume that this