As Ken suggested make sure you have overwritable media .
The media set assigned to the backup Is Keep data infinitely do not allow overwrite---this will not allow the bckup job to overwrite.. So the better option would be to configure a media set which proper overwrite protection period and append period so the media rotates automatically ,so that you dont run into a situvation where in your disk or tape gets full .
Have look on the following document http://support.veritas.com/docs/192265
this will help you understand how media mangement works .