Dell Autoloader PV-124T backup failed, ask for drive clean continue while drive cleaned before taking backup

hello,

 Dell Poweredge r510 server and Dell Autoloader PV-124T, I made Poweredge r510 to master server and  media server, I installed symantec netbackup 7.6 and create a backup policy, When  start the  backup  was failed, and  tape ask for cleaning countinously.

Please give me solution,

I am new to netbackup if u have any documention for netbackup  please help.