sliding door won't close

The sliding door on my 94 caravan won't shut properly. I know the
latch mechanism (the one that engages the pin) is broken, but I think
that's not the real problem. The stripes that run down the side of
the car don't line up anymore. It's like the side door has dropped
maybe 1/8" - 1/4" preventing the lock from engaging the pin correctly.
I had to adjust the pin lower to compensate. I inspected the rails
and wheels any everything and it all looks correct, i.e. no missing
plastic coatings on wheels or whatever. I can't spot anything
anywhere that would make the door drop like that. Has anyone else had
this problem?

brian