Backup Failure

On an NT server running Arcserve IT v.6.61 with a 12/24 GB DDS3 Internal tape drive, every time a backup job runs it fails. Looking at the activity log it is asking for a second tape. However there is no way that the directories it is backing up are filling the tape. Sometimes it halts at 99%, sometimes at 40% I have tried recreating the job many times. Help Me!