A little explanation. Stereo sound has Two channels … left and right … so your output from whatever you’re mixing etc, will leave the soundcard on two channels.
Your headphones will also play in stereo… so that’s another two (different) channels of audio coming from the soundcard… so far then, we’re using 4 channels of a sound card.
Windows can think of those four channels as four speakers dotted around a room eg: one in each corner. Or windows can think of those speakers as being either side of your TV, one on top of the TV and one under the tv running bass only… or windowscab think of those four channels being a left speaker, a right speaker, and a pair of headphones… or all sorts of other combos.
Pick the wrong combo and your sound could go all over the place, or no-where.
Check in the DJ software that it’s only using the soundcard that you’re intending it to use… not the lappys own sound etc.
Also check in the lappy to see if it’s got built-in soundcard. If so, try disabling that lappy built in sound card