If the game doesn't support Dolby Digital 5.1 but is sending a digital signal, sometimes it will just send stuff through your left and right channel and not even your center, let alone your rears.
My guess, is like Pro said, the game doesn't support 5.1. You'll find as you play more games, that this is the case with a ton of games, and the best option is tell your receiver to use Pro Logic II instead for those games (as they usually support that just fine).