Check all your Traktor options:
Select DJM850 on audio setup tab;
Select External mixer on output routing - on the channels select:
Channel A - DJM 850 out 0 / DJM 850 out 1;
Channel B - DJM 850 out 2 / DJM 850 out 3;
Channel C - DJM 850 out 4 / DJM 850 out 5;
Channel D - DJM 850 out 6 / DJM 850 out 7;
(Or select the channels to your choice)
Select USB on the channels input selector on the mixer.
It should work!