The fine tuning adjustment affects the mix based upon the amount of mixing you've set up in your curve. If the slave channel only moves 20% of the movement of the master channel, then the fine tuning can only move that 20% if you have it set up to 100%. The fine tuning cannot add movement to the mix. It can only reduce it. If you want more tuning range, then you'll need to set the Rate A and Rate B to a higher value. Then the fine tuning adjustment can reduce the amount of mix from there.