Roof hatch problem

Hi all
We have the small windup roof hatch over our bed at the front of the van. During strong winds (& dust) the vent opens by itself and thus creates a vacume which sucks in dust through the rear door, despite having the gas vent on the door covered with felt. Can anyone advise how to stop the roof hatch from self opening as we are getting very sick of having to clean out our van after every dirt road. Humf!!!

Cheers Jack