Could be your headphone jack is not properly connected to the socket. Sometimes with new headphones sockets you have to push a little harder to insert the jack properly. If it isn't inserted full length you have a mono signal.
If that doesn't help, try your headphones on a different system, just to make sure it's not your headphones that cause the problem. Also check if the small to big jack adapter of your headphones is connected properly.
If this all doesn't help, there might be a problem with the headphones jack of your interface in which case you should return the interface.
Hope that helps...
