The Mixer knob allows you to blend your input signals with the playback streams from your computer. This allows you to monitor your input signal with zero latency. If the knob is positioned at 12 o’clock, the input signal and the playback stream will be equally balanced. Turning the knob to the left will increase the level of the input signal relative to the playback stream; turning to the right will increase the level of the playback stream relative to the input signal.
If the issue remains, please create a ticket to technical support.
Here is a link to create a ticket: http://support.presonus.com/hc/en-us/requests/new