One of the back seat air vents on the side of the fuselage opens every time I fly. I have resorted to taping it shut from the outside. If this has happened with others, what was your fix?
I would think you could tighten the bolt/nut that holds the part that slides during operation, to give it more friction, and hold it in place better (i.e. open or closed).
Tip - There should be plastic washers above and below the sliding tab with the slot in it. If these are missing, it will be difficult to adjust the screw tension to get the friction set just right.