New Files Are Not Backing Up... Help please!

A few months ago I backed up all files to the WD My Book Essential. Everything worked great.

I have not connected the external drive to my PC since.

I decided it was time to re-back up my files since I have many new files since my last back up.

Over 24 hours ago I connected the external drive to my PC. Now the BACKUP tab in WD Smartware indicates that my new files have not been saved to the external drive. There are a smaller number of files and fewer GB on the external drive (vs the PC). Also some of the file categories (i.e. Pictures) have yellow lines under the PC column in the software indicating the files have not been transferred to the back up external drive.

The software will not allow me to click on the “Run Backup” button as if it is currently running, but the top of the screen says “Ready to Perform Backup”.

What am I doing wrong?

Thanks a million in advance for any thoughts or suggestions.