Your door switch assy has cracked and moved too far away from the door lock for it to engage. You can buy a new one for £34 or so. Or take it out and drill the holes right through and use reassessed bolts to hold the assy in place instead, with very large washers to take the strain. Note, make sure the control cable isn't snagged behind the switch assy on re-assembly or it will still not sit flush -note this is a very common issue and should not be ignored.
David
February 2012