BSTNK_VF not appearing in VBRK

Hi

My issue is that Customer PO field is filled in Credit memo line item. But the PO number is visible in some case but not visible in some other cases in VBRK field. The G2 document is created with reference to CR. I checked VTFA settings. It has A maintained in Reference number and B in Assignment number.But still the PO number is not appearing in some cases.