Crossfader not working(hopefully solved)

Hi, firstly - my set up used to work fine. I am using Cross 2.4 and the U-Mix Controller 1. Now my crossfader is unresponsive. If I'm playing a track in Channel A (or indeed B), it doesn't matter if the crossfader is fully to the left or the right, the track still plays at full volume. No other problems - the volume faders for each channel work and I can monitor the tune that isn't playing aloud through my headphones, all knobs and buttons work.... just the crossfader! Help!