Common problem with these doors is the bolts that come out to lock door to the frame start to rust and squeak and can stick.
You need to open door then lock fully whilst open, then clean the extended bolts with WD40 or simular to clean rust and lubricate, there is also a bolt that protrudes from the top of the door.
Once cleaned and lubricated and the lock also, put oil on key and lock from inside and out you will then find you can spin the key to lock.
Basically the locks are not used enough?