Why is the Gain Knob Not Working?
The Gain Knob is directly tied to the Gain Staging and the Gain Staging takes precedence over the Gain Knob. Meaning if the Gain Staging is set to 50% the Gain Knob will not work for any settings below 50%. This is the same in the 100% setting, if the Gain Stage is set to 100% the Gain Knob will not reduce the Gain at all.
If manual adjustment is desired make sure to set the Gain Staging to its lowest setting 25% (White indicating LED).