I have a '94 MX-6 and have found that the drivers door locks do wear out in time...faster than the passenger door lock,for obvious reasons. Door locks are not hard to replace nor that expensive, but the best solution really is to take it to a Mazda dealer and let them replace the lock, as they can cut the key to match and you won't need 2 different keys. Once the lock is replaced and working,don't use the key to lock the drivers door.
Instead,close (push in) the upper part of the inside door handle to lock the door,while holding the door open...the alarm light on the dash will light up when you do this.Then hold the outside door handle all the way up and shut the door. Then release the door handle. This will lock the doors without using the key in the drivers side lock,and it will last a lot longer that way. I always lock my doors this way,and the drivers side lock(and the key) have lasted for many years now without needing to be replaced.
Also be careful with the key after usng it for some time...the Mazda key blanks are not as strong as they could be,and repeated twisting of the key when locking and unlocking the door can cause stress cracks to appear in the smallest cross-section of the metal of the key. This can lead to the key snapping off. You want to replace the key if you notice these cracks developing...breaking off the key in the door lock would be bad enough,but you REALLY don't want that key to break off in the IGNITION LOCK!