Has it just started doing this or is it a new system that has always done it?
If is has just started then you may have popped somthing in the amp or speaker - try a different speaker to see if you have blown the amp.
If it has always done it, check the test tone on the amp and hike up the levels for the centre channel.
Beg steal or borrow a level meter, I got one from Keene for about £20 I think - they are great.
I thought I had my levels set pretty good till I recalibrated with a level meter - wow, what a difference. I was surpised at how low I had my rears and centre set.
It has really brough movies to life now.