Issue panning - I can't.

I'm having a similar issue with panning. I'm on a Macbook Pro, using an Onyx Blackjack. I've tried some of the things in this thread. The master track is set to center and 100% width. In Garageband, I set my inputs and outputs separately, and when I set the outputs to the default system outputs, it pans just fine. But in Reaper, you have to use the same device for both inputs and outputs, oddly enough, so I can't pan while recording. However, after I record something, If I listen to it through the computer audio outputs, it pans just fine. Any ideas?

In case it helps somebody ("solved") 

My case was probably super evident but I'm new to this. My problem was I had, in the Route of the master, the Out both to L and to R, in two separate hardware outputs, both with Stereo (1/2) source selected. I've deleted them all and created only one with Stereo source (1/2) to "Out L/Out R". I have yet to read about routing to really understand what I've done and what it implies, but I can pan now.