I think K.I.S.S nailed the real problem back in post 2.
If you look at your datasheet for the SDC series, you see that each knob runs two switches. A NO and a NC switch. If you have the bottom two pins tied together, and your circuit is connected to the top two pins, then it will behave exactly like you describe. The combined switch will only be closed when the knob is somewhere in the middle when the NO switch has just started to close, while the NC switch is just about to open. But with the combined series switch pair, it will never be closed in either of the end positions.
For your purpose, you want to only use one half of the switch. Connect to two pins that are directly across from each other with 7.6mm spacing, not next to each other with 2.54mm spacing.