They get stuck. Oil the two slides next to the cams and adjacent movements. Don't be afraid to use too much sewing machine oil. Let it soak. Repeat, and keep trying the selector...gently. The top knob pushes in and twists down, the inner selector is pulled. DO NOT FORCE. It will, over a day or two loosen up. Then mop up oil and you're good to go. PS...oil everything that seems to move mechanically. The machine is fairly indestructible.