It may be the case that you have miss-judged how the USB AD/DA works on these mixers. Take a look at the image below. The Red buttons highlighted are important controls when using this mixer with the USB connection.
In order to record your tracks from the mixer to the computer via USB you need to push the 'USB/2-TR TO MAIN' button into the 'up' position, this routes the stereo main mix channel to the computer via the USB. In this mode you will not be able to hear the playback from your computer in the main mix (this would cause a feedback loop of course), so if you want to hear the computer playback (for instance a backing track playing on your computer) and record at the same time you will need to set the 'USB/2-TR' red button located in the 'SOURCE' section. This will send the USB (computer) playback signal to the CTRL RM output/bus and also to the Headphones.
In order to listen to the computer playback just push the red 'USB/2-TR TO MAIN' button into the down position, this routes the computer audio to the main mix channel. Of course this cancels the routing of the Main Mix to the USB bus, you can only use one routing mode or the other to prevent a feedback loop.
*The exact same method applies for the 2TR bus as well, it reacts the same as the USB bus*