Halo 3 campaign record not updating, no achievments?

I just beat the mission, "The Covennant" on Normal difficulty and after all the cutscenes i didnt get a achievment, i went and looked in the campaign menu and it lets me go to the next mission but on each checkpoint for the mission, The Covenant, i dont get the shield that shows that i have completed it, does this mean that i have to play the mission again and does anyone know how to avoid this from happening again