filter Control issues on SX

I have a ddj Sx barely used, on a normal set up, the filter knobs are all supposed to be in the center, then u turn left and right for LPF and HPF, but lately when the knob is at d center, the sound from channel 1 and 2 becomes flat, u'll need to turn the filter knob all the way 8o'clock (LPF) for u to get a clear sound (as when the knob is in the center). Channel 3 and 4 works fine for now. Pls how do i fix this, a friend of mine using Sx also has same issues on all four channels