Go to the sign in screen be holding out your left arm. it may say you are signed in already. Sign in again anyway, let it track you. It will say face sensor. Go back to game it should recognize you now.
Also out of the game have your kinect scan you a few times in different lighting. It takes a while but is worth it.
I hear so much about this issue, but in over 4 hours of play, I have only had this occur ONCE, and it was when I stepped out of frame in the middle of a Rancor Rampage game without pausing. When I came back in, it had me raise my hand and signed me in as Guest.
If you are using Kinect ID (and perform the setup properly), then you should not have this issue. I think it has something to do with the 'jump in' and 'jump out' format that the game utilizes for it's local multiplayer. --- It's all or nothing. And nothing is all I ever get. Gamer Tag: Johnny Sinister