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

Crossfader Assign may not be set the way you want the crossfader to work.

Select a setting other than THRU for the switches below the channel faders.

Select A for channels you want to hear when the crossfader is on the left, and B for channels you want to hear when the crossfader is on the right.
For details, see the Operating Instructions > [Part names and functions] > [Mixer/effect section].

You can read the Operating Instructions [here].