Confirm Lock Core is the issue
Remove the lock core from the shell by removing the small nut on the back of the lock along with the cam. (she shell is the part of the lock that is affixed to the skin of the aircraft). Once the core is removed put pressure on the brass bar located at 3 o'clock if the key was at 6. The brass bar should have some give to it, but will not go flush. Insert the key and be sure that the brass bar does go flush.
If it does go flush, the there is something else wrong, not the core.
If it does not go flush, please contact us for next steps.
Cause-tumblers sticking due to dirt in lock
Spray Fluid Film / 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 shell and soak in a cleaner. To remove the lock from the shell, remove the small nut, lock tab, cam and stop washer from the back. If the lock was a lock crush washer instead of a lock tab, do not reuse, replace it.