What settings are you using on your amp when playing the different sounds? I've noticed that some concert DVDs use either 2-channel dolby or stero as the default sound format, and so my my amp into 2-channel mode, hence 5-channel stereo, or DPLII, etc (and hence comes from all speakers). Films tend to have a default of DD 5.1 and so the amp flicks into DD mode.
I also found that selecting the DD5.1 soundtrack for the concert DVD then enabled the amp to pick up the DD signal correctly.
Balders.