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. [rekordbox]

Crossfader assign may not be set the way you want the crossfader to work.
In the general setting, channels [1] and [3] should be set to the left side of the crossfader and channels [2] and [4] should be set to the right.
*Crossfader assign sets the channel output to either the left or right side of the crossfader. If you don't want to use the crossfader, you can change the setting to disable it.

For details, refer to "Mixer panel" in the rekordbox Operating Instructions.
You can read the rekordbox Operating Instructions [here].