Quite possibly someone or something got caught on the door latch, or someone fiddled with it while it was open. Apply a small screwdriver to the latching finger while pulling on the door handle. The lock fingers are like a C- when open, they point the open end toward the striker pin (the part fixed in the car's door frame). When closed up, the C rotates around to hold the door closed. Make sure the C is not flipped the wrong way. If stiff, the C may not flip back under it's own power. Flip it the right way, then use something like WD40 to CLEAN it, some heavier oil or grease to keep it working. I like white lithium grease personally, and that's now available as a spray at autoparts stores. Very handy.

Good luck.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 234K, '82 245T/M46-182K, '84 242DL/AW70-100K, '89 244DL/AW70- 212K and I miss my: 86 244DL 215K, 87 244DL 239K, 88 744GLE 233K, 88 244GL 147K, 91 244 183K