A liitle help...

Reach keeps making me (normal mode) repeat Winter Contigency from the beginning. I play through, watch Emile get the link up, watch my stats improve, and then am put right back at the beginning of Winter Contigency. Can’t move on to Alpha, or am I missing something or hitting the wrong button. Even if I do a save and quit before end scene I get put right back at that same beginning. I’m relatively new to live so please bare with me…always played alone but would like to get a better gaming experience. Anybody?