There are two things that could cause the appearance that your touch-faders aren't working properly. If the range seems less than it should be, the faders need to be calibrated. The second situation is when the touch-faders have a different appearance from each other.
1. The touch-faders appear to have limited range. You can fix this by running the slider calibration routine. This can easily be done by pressing the System button and scrolling to the option "Calibrate Slider Position 1 and 2". Press Next and follow the on screen instructions. Your touch-faders should be working properly after the calibration is complete.
2. If the touch-faders have a different appearance...Maybe one slider has the 2 center LED's lit and the other slider does not:
This is a feature, and it means the polarity on touch-fader 1 is set to "+" where the range is from low to high, whereas the polarity on touch-fader 2 is set to "+/-" where the range is positive or negative out from the center. The slider polarity can be assigned by pressing the Shift button, then either of the Playback buttons in the Real Time FX section. The Playback button's shifted function is Assign, which when pressed will show you the assignment screen:
Notice the second category from the left is Source. You can see slider position 1 is set to "+" and slider position 2 is set to "+/-". You can assign the touch-faders for any beat in 16 Beats mode, or any sound in 16 Sounds mode. When you're done press the Events, Pads, or Sounds buttons in the Screens section to exit the Real Time FX assignment.