alright, so i have one final achievement left to get on DOTM but can't. this achievement is the Completionist. i've managed to get it on other xboxes (my brother's and cousin's) but not on mine. i've managed to shoot every single symbol except ONE. i'm assuming it's the symbol in the catacombs on the Megatron level. at first i though it was just the disk until i got the achievement on another xbox. so it's my xbox that's being retarded. any thoughts?