BAPI_PO_CREATE1 - Memory Issue

I'm using BAPI_PO_CREATE1 to create  from a 3rd Party System.

The problem i am having is if we have mutiple PO"s for a Same Vendor or a Cost object  and we do any changes for the Master, then these changes are not reflected.

e.g  . We had a 2 PO Created for a  Same Vendor . the first PO was interfaced and it was sucessfully created in SAP, Then we Blocked the Vendor but still the 2nd PO was created in SAP.

Plz any help will be very appreciated.