Unable to make backup with No overwrite media set using tape library

Hello,

I'm running Backup Exec 2012 and I am having some problems creating backup jobs to backup data for offsite storage. My full and differential backups works without problems using scratch media in a tape library. But when I try change the media set from my daily or weekly media sets to a media set with overwrite period infinite, the job fails with "No appendable media could be mounted".

What should I do to troubleshoot this error?

Thanks