Today the power door locks have started to play up, I can unlock both doors with the key but when I lock the doors only the drivers door locks, this leaves the passenger door and boot unlocked.

Any ideas on what could be causing this to happen.

 

More than likely a dodgy drivers door lock actuator.

 

I spoke to the garage I use and they said the same thing. I have read online that it can get bent and some times it just needs straightening out.

 

I have now replaced both the drivers and passengers door locks/actuators and am still having problems.
From the drivers side I can unlock the nearside and boot but cannot lock them from the drivers side.
From the nearside I can only unlock the nearside.
Does something have to be reset if so does anyone know what the process is to reset it

 

Funny you should say that, I have had my MX3 for about 6 months, I have a similar issue.
The boot is separate and can only be unlocked with the lock itself, or the release in the car.
The passenger side locks and unlocks both sides.
The driver's side unlocks both, but only locks the driver's side.

 

Mine turned out to be the power door lock relay which was a nightmare to find, it was up behind the glove box and behind the blower motor. It has a printed circuit and part of the circuit had broken, I bridged it and it's all working fine.