When you lock the cap, what is supposed to happen is that the chrome top spins freely and does not engage the rest of the mechanism. I would try to figure out why this is not working on your cap. Maybe with some penetrant into the lock cylinder, and the rest of the cap, to try and get the locking mechanism back 'in action'. If that fails, try taking it to a locksmith to see if they can fix this. New caps are pricey these days I believe.