At the risk of sounding like Capt. Obvious

, this seems like a classic case of unbalanced channels. Your left & right speakers may just be louder than your center speaker. When was the last time you balanced the sound levels coming out of all the channels in your HT system? Often people refer to this as "calibrating" their system, but it isn't really that complicated.
If you have an older HT receiver, you need a sound pressure level meter (
Radio Shack sells them) to adjust all the channels to the same volume. I find it works much better with the SPL meter than to do it by ear. Newer HT receivers have various ways to do this automatically, but with varying success.