enterprise vault 10 - back up with back up exec 2010 r3 problem

we have been successfully backing up our EV environment every weekend using back up exec 2010 r3

the back up job goes through verification stage but finishes with an error

"Full_EVBackup -- The job failed with the following error: Backup Exec cannot place the Enterprise Vault vault stores or index locations into backup mode. Ensure that the selection is valid for a backup operation, and then try the job again."

so far I have restarted backup exec services and retried job but i get the same error

can anybody assist

thanks

Lois