Ever since I got my 1984 300TD-T the doors have to be SLAMMED to close completely (I had 300SD and 1980 300TD years ago that latched perfectly with no effort at all) . I replaced the weather stripping on my drivers door and it got even worse. Luckily, the 4 bolts that hold the striker plate on were very easy to remove and didn't strip so I played around with it's positioning, but that didn't work. I then replaced the entire door lock striker with a brand new one. It now takes even more effort to have the door close completely. It latches into the first catch effortlessly, but not into the second. I've tried numerous different positions of the striker and the weather stripping is also on perfectly.
I have noticed that when it closes in the first position, you can see that the door sits a hair lower than the rear door until you close it all the way. I'm sure this has something to do with it too, just don't know how to remedy it.

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Current fleet-
2015 BMW 750i xDrive
2009 BMW 535i xDrive
2004 Chevy Avalanche 4x4
1993 Mercedes 300SD
1993 Mercedes 300SE (parts car)

Previous Benz's-
2001 Mercedes SL500
1983 Mercedes 300SD
1980 Mercedes 300TD
1996 Mercedes E300D
1999 E300DT
1984 300TD-T
1988 Mercedes 560SEL (parts car)
1979 300SD
2003 E500
2004 S430 4Matic
2001 E55
2003 E430