Recieving errors when trying to back up Exchange 2007 Mail Stores with Backup Exec 2010

I have two jobs that run incremental every hour on my Backup Exec 2010 server that backs up two mail stores I have on my Exchange 2007 server. sometimes the jobs run fine but most of the time they just fail with this error. anyone else have issues getting there Exchange 2007 stores backed up?

Job name            : vs-app-mail Unlimited Store-MAILHUS - Exchange Backup High Priority-Incremental Transaction Logs
Job type            : Backup
Job status          : Failed
Job log             : C:\Program Files\Symantec\Backup Exec\Data\BEX_SV20124B_02362.xm
lServer name         : SV20124BSelection list name : vs-app-mail Unlimited Store
Device name         : Daily 1
Target name         : Daily Disk
Media set name      : Daily Full Disk Backup
Error category    : Resource Errors
Error             : e000037b - Using the job settings, Backup Exec attempted to back up the passive copy of the Exchange database. However, the passive copy was not available after the Microsoft Volume Shadow Copy Service (VSS) snapshot was performed. Try to run the job again.
For additional information regarding this error refer to link V-79-57344-891