File size on my computer does not match file size shown on Passport - so files not backed up?

My computer shows 149 GB in Pictures, but after backing up, the Passport only shows 91 GB. Just updated software, done a back up, tried File Backup and Category Backup. Same thing. Why is the Passport not backing up everything in my Pictures folder (that’s on the computer)? Thanks ahead of time for any advice.

If you are using WD smartware. click on the option to stop the backup and restart the computer and click on start backup again to see if the software resumes the backup.

Tried that already. Still not backing all the files under “Pictures”.