I'm no 'expert' but...
Is the vacuum sitting down? On mine, the dial a nap dial only clicks if the base of the vacuum cleaner is touching the floor. The axle the two wheels are attached to is what causes the 'click,' there's a bent portion of it in the center that touches the dial-a-nap dial, and if it isn't pushing against the dial the dial rotates freely. Really simple but smart mechanical design if you ask me.