It's not a lump of ice / snow in the runner is it ? (just a thought)
I cleaned all the runners on mine a few months ago, when it started getting stiff on closing.
I found the best way to get all the old manky hardened grease out was to use an old towel liberally soaked in paraffin.
This left a nice anti corrosion type coating on the surfaces too

A messy job, but the amount of cack that came out was obviously going to make matters better.
I then re-greased all the runners, this took a heck of a lot more grease than I anticipated, I had to go and buy another tub of it !
All now works nice and smoothly.
Good luck with it
COURGETTE wrote:mine sometimes bounces back open, id be interested to see these replies
Is yours the later type where the catch is at the back of the door ?
If so mine used to do this (when I first bought it 5+ years ago).
I had to adjust the catch mechanism, quite easy to see the simple "peg" that sticks out from the van body (not the door) and this can have it's position adjusted by slackening off the nut (easily seen) and adjusting position, then tightening up. The catch on the back of the door can be adjusted for position too if need be, but for this you need a LARGE STRONG cross point screwdriver !