I posted this fix in another thread, but could not find it.
If the rubber is stiff due to age, the only thing I have done that worked was.....
Tape a piece of waxed paper onto the metal of the wing. Run a thin bead of black RTV on the rubber. Close the wing but do not latch it completely. Just enough to put pressure on the RTV. Let it sit until the RTV sets up and you should be good to go.