Door doesn't lock, except with remote
 

Hello good people,
I have a 2008 Mazda 3, I still can lock/unlock the doors with my fob normally. But not with the mechanical door lock on the driver's door.
(the other doors are fine, but each locks itself only of course).
Note that although most of the time it doesn't work, sometimes it does.

I know my question might sound too simple to some of you, but I'm really terrible with handy stuff.
So really any help is much appreciated, especially if it is simplified step by step.
:suitdance