Delivery note number not printing all the time

Has anyone come across this before....

Clients convert deliveries to invoices via batch processing. Out of around 500, a few of the PDF images dont show the delivery note number in the relevant del note field. THere is a del note number in IHEAD & if you repinrt this via SOP, it publishes it, just the PDF taken at the time of conversion doesnt show this.

Any ideas?

Many thanks

James