I can't say exactly what the problem is, but can give you something to think about. I had a similar problem a few years ago with the right (passenger side) front door on my car.

A body shop was repairing a dent in the door, and I had told them about the problem with closing the door., which was already present before the door had been dented. They tried adjusting the door catch/striker and so on, and told me that the latch assembly was broken. It required replacing the latch assembly. They got a replacement from a junkyard. It's been working fine since.

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Marshall Welch
Seattle, WA
1982 300D-T