How to repair the rear left door lock

Today my laptop was stolen because of the rear left door will not lock. I would appreciate it if I could fix this problem right away. Its my mums car and my laptop was stolen because of negligence to this problem. So please sirs help me out with a step by step tutorial because next time they might decide to take the car.

What happens is when you lock the doors the lock jiggles as if its try really hard to obey the locking command but it can not. Occasionally it would lock which tells me that it can be repaired and that it might just be something that needs to be corrected.

Please and thank you
Ejay010