Exchange mailboxes of domain admin users not included in backup

I am using Backup Exec 2010 R3 on Windows Server 2003 Standard with Dell PowerVault 124T tape changer.  I have Microsoft Exchange Server 2003 running on Windows Server 2003 Standard.  I have three users (including myself) with domain admin, enterprise admin privileges (full privileges).  All user mailboxes are backed up just fine and I can see all mailbox folders and emails in the backup list.  However, it does NOT backup the three users with full privileges.  The name is listed so it saw the mailbox there, but nothing is in the Inbox when trying to restore.  It is only these three mailboxes that are missing the content on the backup.

Any idea why these aren't being backed up?