LTO Ultrium Tape drive in 2003 SB server not restoring

We have a LTO Ultrium tape drive running with
NTbackup with week and month tapes.
We need to restore some data which is on the tapes from januar end february 2008.
In the restore config we find exactly what we want to restore, but when we click continue, the software tells us ‘waiting for the media to come free’ and nothing (seems to) happen.