When I look at the details of a backup there is a long list of files it says were not backed up. The file names are long lists of numbers and letters with no meaning.
What is the name of the program you are using to manage your backups? Your WD My Book unit does not run automatic backups without a dedicated application handling the process.
Additionally, try manually copying some of those files directly into the root of your unit. If files can not be copied then the location of those files exceed file directly limits and parameters on path length.
I’m using the backup program that came with the WD drive - WD Backup.
In terms of copying them into the root: I don’t know where to find the files. Their names show up in the backup detail but you cant open them or get any information on them. The WD Backup utility doesn’t give any path, just a very long, 2-line file name for each file, and there are about 60 of them. I just ordered Acronis True Image 2017 backup program, which is highly rated, to see if it works.
I’m suggesting a LongPathTool program for your long list of files.