Can´t create iscsi snapshot

[Manu]

Hi,
i´ve created an logical volume 465GB!
Now i wan´t to create a 465GB snapshot in this volumegroup !
i choose New Snapshot ,give 465Gb of the free place to them and check accept an then:
nothing happens - no snapshot, 465GB free place und no error appers on the screen!

Please Help, i try backup our new iscsi volume and i need the snapshot for this procedure!

Thanks Manu