WIll not backup outlook files

My WD Backup program has started having a problem backing up one of my outlook .pst files. I have two outlook accounts, one backs up no problem, the other will not backup about 75% of the time, message just says cannot back up. Why is this happening? never happened until very recentlly.

Hi,

You could refer to the following link: https://support-en.wd.com/app/answers/detail/a_id/17455