Backup Utility Error

Using 2000 Servers backup utility and a DDS tape media, I have manually backed up the My Documents folder onto a blank tape. This was successful. But when I schedule a full backup to take place that night, I recieve a report log that says “Operation was not performed because no unused media was available”. I will be attempting a full backup manually tomorrow at the end of the day to see if that works. If anyone has any other thoughts,or has seen this message, I would appreciate your thoughts.