Scheduled Backup using Windows 2000

every time i set up an scheduled backup using the exisitng backup program in windows 2000 and an internal dds4 scsi backup unit, it sets the media to idle and allocated not idle and available when it finishes the backup. when it goes to run the next night it says that there is no unsed media available so consequently the backup doesn’t run. Have tried seting it to use an existing media set instead of new media, but thne it asks to load that media set tape the following night. Thank you for anyadvice in advance.