I have a 87 420sel and for the last few weeks I haven't been able to close the drivers door without having to slam it hard to close it tightly. Just using normal force will not close tightly. any idea? Is it something inside the door assembly or the exposed locking mechanism?

The piece that is attached to the body is bad. It has a plastic piece that falls apart and causes the latch not to catch the second position.

door isn't closing
 

I'm no mechanic and just learning the benz, but I had the same problem on my recently purchased 380se,(only one of the reasons they sold it). But the same day I got it home,I cleaned and oiled the mechanisms using a silicon lubricant and voila! Closes perfectly.