library insert. Doesn't jump to next tape

Hi,

I have Backup Exec 2012 SP4 that backups onto a HP autoloader.
Last night it was backing up on of our servers and reached the end of the tape halfway through the backup.

Normally it should jump to one of the other available tapes (there were 3 others in the mediaset that were completely empty), but it didn't.

It got stuck on "library insert" for the next 8 hours. When I cancelled the backup, the next backup began without interaction needed, and a new tape was loaded.

Anybody got an idea why this happened?