How many PC driver outputs pairs appear for that mixer? Other digital mixers I've seen have had an abundance. There may be a routing table to populate on the mixer (on the Berringer I am currently using, you can route any input to any channel) - or it could be arriving to the mixer and you just don't know how to make it audible (on that same mixer, the channels switch between USB return or analog input). Anyway I would be surprised (but not totally) if one half works and the other doesn't.

It has been my experience that the more features a device has, the more time you must spend with the manual...