Purchasing partner determination - higher level indicator no effect

hi all,

I have setup the Partner Schema for Purchasing Document with just VN and CP and tick the checkbox for Higher Level for both. Then I created the vendor master with 3 diff CP and no data at the plant or VSR level. When I create a RFQ and enter this vendor code before entering any line item, the dialog popup to choose the CP appears as expected. However if I enter the line item first (including the plant code) before entering the vendor code, the dialog popup does not appear. It seems that partner determination is not taking place at the POrg level despite the Higher Level indicator being ticked. I am on ECC6. Could any one help ? Thanks.

Regards

ET