Two theories come to mind. The first is that one channel is louder than the other. You can test one speaker at a time with an SPL meter, and if it turns out one is louder, then you will have to track down which is the offending component. This can be done by swapping out components one at a time, assuming you have redundant components lying around.
The second theory is room interactions. If your setup is asymmetrical, or if large objects in the room are near one of the speakers, it can cause center imaging problems. I have experienced this in my system. It can usually be solved by addressing the asymmetries. Good luck.
The second theory is room interactions. If your setup is asymmetrical, or if large objects in the room are near one of the speakers, it can cause center imaging problems. I have experienced this in my system. It can usually be solved by addressing the asymmetries. Good luck.