Files missing from Document Copy and not updating in Audit Trail

Good day, 

I update multiple files under one document and every month add a new report with a date, i then hide the files under the files section.

Recently i went to look at a previous file under the document and found one of the files are missing. There was no audit trail and i cannot find the specific months file. when i revert back to the distribution done and the transmittal it opens the latest report and not that specific one that is missing.

Is there a reason for this?

Thank you