Thats the only thing i can think of.
ME and my friend have spent the last 3 days playing online co op on Halo ODST and we finally beat it today. We got the legendary ending and everything. But we do not get the achievement nor does it show in our completed Co op mission list.
We had to continue the mission from a checkpoint and the gate was closed behind us. is there something wrong since we are plannin tomorrow to restart the mission completely