OK if you are in the UK then defiantly try going to http://tape-drive-repair.com/
for them to help as these can be a pain to seat the picker correctly and you might well need parts anyway.
However if you are in the US and are unable to do that:-
Dissconnect the power.
Gently raise and lower the picker by hand and again check for any obstructions.
Sometimes the picker comes off its rails or skips a gear.
You can also try raising the picker to the very top before lowering again - make sure the picker is totally level when it returns to the bottom.
This is the same unit (in effect) as the dell TS4000 so that might also give you another search term when googling the issue.
These units are great libraries but you must be very gentle with them when doing this sort of procedure as it is easy to bend and break things.
Also make sure the library is totally square in the Rack - i have one unit where the customer had one corner 1U too low in the rack but the library had still managed to be screwed into the rack. Because the library was no longer square, things started to jam and fail to pick tapes. Once square again all was well.