Cause-tumblers sticking due to dirt in lock.
Spray Electrical Contact Cleaner in key hole, move key in and out, turn lock, spray more cleaner and continue until lock operates smoothly. Never spray oil lubricants like LPS or WD 40 in the lock, only dry lubricants. In most cases, using electrical cleaner alone will fix the lock. If you still can't get the lock to work, remove it from the handle and shell and soak in a cleaner. To remove the lock from the shell, grind the divots where they pinged the woodruff key, then using a flat blade screwdriver, knock the woodruff key out.