ArcServer v9.01 - Unable to find blank media

We work with Arcserver v9.01, the servers are PE2650 and the backup library is a PV132-T with 2 LTO drivers.

When we run a backup job than need to span to another media, after the fisrt media is full a message appear stating 'Unable to find this media or blank media'. Of course another blank media is available but the job never find it, the only option we have is to cancwl the job.

Why does it can not spannig media?