Djm 700 problem with crossfader? Hopefully...

So i just brought a secondhand djm 700 and it was working fine the first time i used it. The next day i turned it on and thats when the problem started. So pretty much channel one works fine but when i put channel 2 on , the crossfader wont work, so i have tryed everything, put it into channel 3 and 4 and it still doesnt work. Hope that makes sense lol im a just a beginner