Sales Order Invoices not printing to PDF

A client states that they select Print/Electronic file and they do not get any error messages, but if they go to view in Paperless Office, some of the Invoices in the batch do not exist.

I have already asked if someone had already printed the invoice from the Batch prior to printing the batch and therefore they should use Print Invoices already printed.

Any other ideas?  They say it is not consistent.