The crossfader could be dirty, defective or just not connected properly. Remove the top screws, unplug and remove it, blow it out with compressed air or try some switch cleaner. Reconnect and see if it helps.
This is more likely to be an issue with the fader itself rather than the controller.
Then it’s probably defective. I keep the device very clean and free of dirt. When I’m not using it, it’s in a flight case with a decksaver on top. A bad connection seems unlikely because I’ve never removed the crossfader and can hardly imagine it coming loose. So I’d best look for a different crossfader, or perhaps try using a contact cleaner first.