Field XBLNR in EKBE is mostly empty

Hi,

While posting the vendor invoice by MIRO, the reference field XBLNR is mandatory where we mention the vendor invoice #.

I am trying to develop a report from table EKBE which shows all PO's with the vendor invoice #.

But unfortunately, i don't understand why this field XBLNR is mostly empty.

I have tried to get the information form other tables also like EKBZ, RSEG, but it is the same case.

Please help

Regards

SAPXPT