Windows 2008 R2 cannot delete older backups

I have a 2008R2 server running exchange 2007. We are using windows server backup to backup exchange data and local system data to a SCSI attached drive mounted as X:

This drive is now full and it appears that windows backup is doing as expected by deleting the oldest backup automatically, however we would like to delete quite a few of the older backups. I have looked through the following resources for help

http://blogs.technet.com/b/filecab/archive/2009/06/22/backup-version-and-space-management-in-windows-server-backup.aspx

If I try to perform vssadmin Delete Shadows /For=X: /Oldest

I get the error "No items found to satisfy the query".

Am I missing something easy here? Or is there complication due to the fact that the job contains exchange backups as well as system state backups?

Your expert guidance is appreciated!