Under the knob, there's a hole in the side of the body of the front cover with a small coil spring that pushes a tiny ball bearing outwards against the inside of the knob itself. There are small detents on the inner surface of the knob that make the "clicks". In all likelihood, that ball has worn out the inner surface of the knob so that the ball has slipped out and the end of the spring has jammed itself between the knob and the body of the front cover.
If you look on the inside of the front cover, you will see a round gray plate that's pressed over the "fork" on the underside of the knob. There should be six tabs that grip the shaft of the fork. In the past, I've used a small cutting wheel to grind a notch into every other tab until I could bend them away from the shaft, then work that gray plate up off the shaft. Remove the knob, and the ball and spring underneath, replace the knob and press the plate back on. You can still use the Manual Adapter, but it won't have any clicks, so be careful when setting manual speeds.