Hi Paul,
My door recently did the same thing. I think I ended up spraying anti freeze in and then gave it a whack and it shut. But my door was kaput shortly after so I would advise extreme caution on cold mornings! - see my recent thread:
https://www.the75andztclub.co.uk/for...d.php?t=310402
Quite a few posts with pictures from kind forum members that might help you understand the inner workings of the door better.