When I slide the crossfader, the sound doesn’t change. Even if I slide the crossfader all the way to the left/right, I can hear the sound from the other deck. Why is this happening?

The crossfader settings may not be set in the way you want the crossfader to work.

・ Check the crossfader reverse setting.
If the [REVERSE] switch on the front of the DJM-S11 is set to [ON], the operating direction of the crossfader is reversed. In this case, set the switch to [OFF].

・ Check the crossfader THRU setting.
Set [CROSSFADER THRU] to [OFF] in the Utility settings. For details, see the Instruction Manual > [Settings] > [Utility settings] >[PERFORMANCE menu].

You can read the Instruction Manual [here].