Assignment field in Invoice to populate the invoice number SAP SD

We have "Assignment" field in Invoice. This field is to be populated with the invoice number for accounting purpose. I have made the copy control " Assignment" as blank. In the help document it is mentioned as "If the field in VTFA is maintained as E the invoice number is placed in the field of assignment automatically.

It so happens that some times it field is populated with invoice number and some times not. Can you tell me what is the problem? Plrase help.

If you can sugggest any SAP note it would be helpful.

Regards

Nagesh A