Whenever I'm in a mission (STF, episode, whatever) the shooter mode does not work. It'll go to it for a second, and snap right back. But if I'm out of a mission, it works just fine. Any ideas of the cause of this problem?
Some of the UI elements will temporarily bump you from Shooter mode, such as the map and inventory windows. When you close them Shooter mode re-enables.
If you have them open when entering shooter mode you'll see the effect you describe.