n00b question regarding 5.1

i can use my 5.1 for MGS4,so it works perfectly.

however,for blu ray movies, i cannot get 5.1
i only get 2 channels,left and right

anyone know what could be wrong?

a breakdown on what i tried:
1)put in I am legend blu ray disc, played with the audio options-all produce only 2 channel sound

2)took out the blu ray movie,then put in MGS4-speakers worked perfectly

3)put the blu ray movie back in, still no sound from the surround speakers.

4)tried changing the audio system's channel from Audio 1 to Video 2 (and changed the cable as well) but still same. still only 2 channels.

anything i may be missing out?