Seat belt won't extend

[Phalcon51]

I recently bought a used driver's side seat belt to replace my original that is fully extended and won't retract. The new (used) one has just the opposite problem: it's been fully retracted and I can't get it to extend more than about 5-10 mm before the mechanism locks up. Is there any way to release the mechanism and allow the belt to extend without locking up? Otherwise, I need to try to return this one.

Thanks for any help.