Can only record one track from mixer to GB

I am using another computer to supply a pre-recorded track into my Mackie Profx8 mixer. I have the mixer plugged USB into my mac. I set up 2 channels on GB. Only the pre-recorded track will record into GB. The vocals will not. I have panned both the track and the vocals but still only the track records.

Not sure where to go to from here. Any suggestions?