Backup Exec not overwriting

I'm using Veritas Backup Exec to regularly backup a number of folders on one of our servers.  On one server which we call the 'Fileserv', there are 3 folders in particular which are about 100GB each.

-  I have set up a seperate backup job for each folder which backs up to a seperate HDD (permanently attached).

-  If I go into the 'Job Setup', I currently have Backup Exec set to 'Overwrite Media' in the 'Device and Media - When this job begins' section.

-  The backup method selected is 'FULL - Back Up Files - Reset Archive Bit'

However, the overwrite seems to happen.  It seems to just create new backup files to the point where the 500GB HDDs are full.  This only takes 4-5 days given the backup runs daily.  After this, the job fails continuously until I delete the old backup files and free up the space on the destination HDD.

As it stands it requires nurturing on a day-to-day basis and I feel as though I've tried pretty much every logical alternative setting I could find.  Yet, it could be something really simple somewhere!  Any ideas?

Thanks in advance.