Unable to overwrite USB drive for backup

Hi everyone

We are using backup exec 2012 for trial. We are trying to use removable usb drive to backup data daily. Our backup model is to overwrite data daily and hence we dont need historical records. However, i am unable to get backup exec to overwrite data on the tape. It keeps saying out of space even though ive set retention to 1 hour

Ive read that backup exec does not overwrite the latest backup set. is this true?

thanks in advance