Delivery number in QALS table

Hi experts,

I want to know at what circumstances the  field LS_VBELN (delivery) is filled for inspection lot in table QALS. I tested a scenario where Inbound delivery  with HU was created for PO and then posted GR for the Inbound delivery, then inspection lot got created but the delivery number was not assigned, the field QALS-LS_VBELN was blank. 

Please tell me why the inspection lot is not getting delivery number.

Thanks

Danny