Bliest73
Most likely it’s down to the cable,
the cable needs to be a stereo 3.5mm from the headphone output to 2 mono jacks split,
This splits your signal to two channels allowing you to hard pan left and right to maintain the stereo mix of the file
It’s possible another cable or DI is giving you a summed mono or not a full stereo output,
For example a single stereo cable into a mono input on your mixer will not give a stereo sound
Without knowing the specifics I can only assume certain variables
Alternatively if your mixer supports it you can use USB out