Try this mate, :wave:
Open the door. get a screwdriver and place it into the latch mechanism on the door..... from the side you should be able to push the screwdriver into it and cause the latch to lock ito place.....
If it does this and the little lever on the mechanism stays down in the locked position. there is nothing wrong with the latch assembly.
The problem then is more than likely the alignment of the door itself....(eg. hinge pins/ bushs u/s.)
or more commonly it could be that the door striker(bit attached to the b pillar of the car) is not aligned correctly with the latch assy.
To fix this use a large philip head ( star/ cross) screwdriver and loosen the couple of screws that hold the striker in place...... but dont loosen them too much as the striker will keep slipping down lower than what you want.
When it is loose... try and line up the latch and the striker assy as best you can. then move the striker towards the outside of the car a fraction...... ( this ensures that it is just not too far in) retighten the screws.
Try closing the door. :w00t:
Keep trying this until you are happy with the adjustment of the door.....
Lastly.. recheck that you have fully tightened the screws............ and then check that you have enough tension on the door seal rubber.( make sure the door is touching the rubber)
This will ensure you dont end up with any annoying leaks when it rains next. :cry:
Try tha and let us know how it goes!!!
Catchya!