Not all movies have a lot of sound in the surrounds. Does this happen with all movies or only specific ones? On the movies where it happens, have you tried rewinding and relistening? Does it always happen for that scene or just sometimes?
Also, if you list a movie or two and the specific scenes you are noticing this on, others can tell you if that's how it sounds on their systems too. Try watching a scene from a movie with a lot of action in the surrounds to see if it drops out or plays.
Jack