Center imaging problems

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.

Cable out of phase..( one side )???

Often it a room/set up issue. Sometimes it an imbalance in the electronis somewhere, or as likely as that, a hearing imbalance. If when you swap cables from one speaker to the other the voice doesn't shift to the other side you may just have a hearing problem in your left ear or an unbalanced set up for example which can be caused by an open area on the left side and a wall of the right side. Simply shifting speakers or your listening chair as suggested by Rwwear can be a good solution.

Not all recordings will center a vocalist.

True Shipo. That's why a mono recording makes it easier to tell if the image is out of balance.