Roof vents won't stay closed during travel

I've noticed that the two non-motorized roof vents keep coming open during travel. I'm worried that one day they'll be open during a gully washer (like the one I'm in right now at Acadia in ME) and everything inside will get soaked. Has anyone a. had this problem and b. found a simple fix? I'm afraid to torque down too hard on the knobs for fear of breaking them.

Thanks,

Phil