Diagnosis
The switch mechanically adjusts the opening of the drip stop in the brew basket. If the flow is unchanged, the linkage is disconnected or the drip stop itself is stuck open.
How to Fix It
Remove the brew basket. Look at the small black plastic drip stop (the flap) inside. Manually move the switch on the machine and watch for a metal wire moving the flap. If it's disconnected, you can often re-hook it. If the flap is stuck, clean it with vinegar to dissolve any scale.
If That Didn't Work
The internal wire linkage may have broken. This requires replacing the entire brew basket assembly, as the wire is not a separate part.
Heads Up
The half-pot setting doesn't change brew temperature, just flow rate for optimal extraction with less coffee. If it's broken, just use the full-pot setting and a half batch of water—the result is nearly identical.