One thing you can check is to make sure the MONO button is not selected on the mixer. This would not be likely but possible.

What version patch of RB? This is important because for a period of time what you experiencing was experienced by others and a patch fixed it for most. Again, do you have the latest patch for your version?

Another is your Mixer Output section. But that would be pretty low chance.

Make your sound your own!
.. I do not work here, but the benefits are still awesome