Have you tried to adjust touch-sensitivity of your DM? I had a similar problem about year and half ago: one fader got stuck. Just like in your case, it worked fine, but motor didn't move it. Found out, it thought I was having my finger on it (by setting the peak leds to show fader touch). All this because there was a thunderstorm night before and air humidity was extreamly high. Adjusting the touch-sensitivity setting fixed the problem.