VERITAS NOT BACKING UP ON SCHEDULED DAY

MY VERITAS BACKUP EXEC 9.1 RUNNING ON WINDOWS 2003 SERVER FAILS A SCHEDULED JOB WITH ERROR INSERT OVERWRITABLE MEDIA INTO DRIVE. THE MEDIA IS OVERWRITABLE BUT THE JOB STILL FAILS, BUT THEN COMPLETES ON THE FOLLOWING MORNING AFTER ASKING FOR THE PREVIOUS WEEK DAY TAPE(MY MEDIA IS SETUP AS WEEK 1 RUNNING FROM MONDAY TO FRIDAY UP TO WEEK 4.I RUN A FULL BACKUP EVERYDAY, HOW DO I SOLVE THIS PROBLEM