Oracle Purchasing Shipment issue

This is on 11i and r12 both.

Here is the problem scenario.

1. PO Number: 1234

2. PO Line QTY - 2200

3. PO Shipoment QTY - 2000

4. PO Status - Require Reapproval

5. Someone tries to enter remaining 200 on shipment lines.

6.  When selecting the ship to address from the List of Values,  same as shipment line #1(that has qty of 2000), it does not show up on the List of Values and says "Invalid SHIP_TO".

#6 is my problem.  Why shipment line #2 does not have same ship to as line #1? 

Please advise.

Darsh