if its is sticking badly and is actually holding a door shut, it could be that the door or lock were installed badly and there is pressure on the block from the frame, try moving the door into a few different positions and seeing if it gets any easier to turn the lock futher, i.e. lift the door up a bit/ pull it to you/ push it away/ maybe even push down, but remember, te door handle itself may not be that strong, so dont press too hard on it in case it snaps!
as stick said, it could be the control shear line you picked, there could just be some kind of release mechanism, like the tip of the key or something like that to release it.
someone who knows about best locks would be the most useful person to you, but this is something to try out until they reply
i hope this helps, good luck.
~ MiT ~