DJM 900 Nexus 2 right?
Have you tried sending it back to Traktor, setting USB 11/12 as FX SEND on the output tab on the DJM Control Panel?
On the Traktor setup / Input routing , select USB 11/12 on INPUT FX SEND, and on the Output Routing Tab, select one of your channels (ex channel 4 / USB 7/8) on OutputFX Return.
Never did it on the 900nxs...only on the DJM850, but it works about the same way.
I think it doesn't work seding via usb send ....it will only work plugged directly to a IPAD/Similar using software like the rmx1000 app
Sorry for the poor english, i'm from Brazil.
Good luck!