I've noticed this thing you've come across not just with this game, but multiple other games, not just game saves, but xbla games. If you saved your game while you were online, then thereafter you will have to be online. If you saved your game while offline, then you're in the clear to play anywhere, anytime. its stupid but its a way microsoft made to prevent people from "game saving" or trading xbla games even though it is still possible to do. Just an annoyance, not a glitch.