This has been one of the bugs in Eurovans for years. Some people say grease & cleaning work others have adjusted the striker plates. I have done both with limited success and ended up taking it to a body shop where they made some minor adjustments to, I think, the door position. It has sorta worked in that it now closes during winter months without having to slam it multiple times every time. Now every so often it will not shut without a good slam but not as often as before.
Spencer