Are you getting an email with a list of files it couldn't backup, that says "The following files did not backup because they were open or not reachable:"?

If so, check the path to the files.  I was getting this error for months and I finally found the reason a few days ago.  Data Protect fails to backup files with a path longer than 256 characters.  Very stupid issue.  The entire path (C:\Users\Username\Documents\Folder Name\Sub Folder1\Sub Folder2\Filename.txt) has to be less than (maybe equal to or less than, I didn't check) 256 characters.

Also with one of the updates in the last 5 or so months Data Protect won't backup the company file for me if QB is open, so now I have to close it when I'm done for the day to get a complete backup.