If the SS main dial is set to A:=Auto, you can't change it using the front or the back dials. Set the SS main dial to C:=command, and then the front of the back dial should work. And notice that when the SS main dial is set to a specific value, which ever of the front of the back dial that you have set to modify SS will only shift the value slightly, try it. In my opinion, this is an amazing feature, because it allows the user to slightly adjust without the need to change the main dial if needed.
I hope this helps, let me know if it doesn't.