DJM 900 NXS - Crossfader Issue (Not Working)

Hi guys,

I bought a second hand DJM 900 Nxs last mounth and I noticed today that the crossfader is not working. When I set the 1st ch to A and the 4th to B, only the B is playing, the A is muted. I can move the cross fade side to side and nothing happends. Does anyone knows what can it be? Thanks!