Missing reference number in Indonesia invoices

Hi all,

for Indonesia reference number VBRK-XBLNR is generated through number ranges.

Official Document Numbering - Indonesia - SAP Library

but in some invoices, reference number is not filled.

Number range was incremented when invoice was created, but reference number  VBRK-XBLNR is empty.

Also when i cancelled invoice (error : ref number not present occurs. so i updated vbrk - xblnr manually and cancelled)

and created new invoice for the same delivery, reference number got filled

i want to know

1. what are the fields need to be checked

2. Why number range is incremented but not filled in reference field in invoice.

thanks in advance,

Pradeep M.