Ford Falcon Forte AU III?

I have a 2001 ford falcon forte AU III. I bought it in January last year and it is a brilliant car. However, I now have a problem with the driver side front door. When I use the remote keypad it only unlocks and locks the three penger doors. Also when I try to lock my car, it beeps as though there is a door open. I can use the ignition key in the door lock, but this only opens the drivers door and then I need to use the on near the steering wheel to unlock the other doors, and this is tricky while trying to keep 2 toddlers away from the road. Does anyone know why this iis happening and how it can be fixed?