Is this happening for all cartridges, or just one?  All tape drives, or
just one?  I've seen some carts get hung up in the drive mechanism.
 Sometime the fault of the cartridge, sometimes the drive.

Latest microcode in the drive, robot?  It also seems that perhaps the
unload command isn't getting processed.  Once a cart is dismounted, the
robot should open the door and remove the cartridge.  This isn't happening.
 See the first comments above, or there is something wrong in the software.

As an alternative, use the checkout libvol libname volname checklabel=no.
 Since the carts are bar-coded, you really don't need to check the physical
label.  This will also save time and the cartridge in question should be
moved from a slot to the CAP immediately.

I'd focus on a particular cartridge or drive and see if the mechanism is
fouled.  I haven't fooled with the STK that much so I really don't know how
the mechanism works.  You might try fooling the robot door interlock so you
can watch what happens after the cart is dismounted.  Does the robot try to
lift the arm without success?

I hope some of this helps!

Kelly