Hopefully you've got the issue nipped in the bud, but in case you didn't, have you used an SPL meter to ensure both speakers are level matched? If one is even 0.5 dB louder it can cause issues. I had to push my right speaker back 5 inches further than the left speaker (I use a preamp with no level controls) because the side-wall was causing the right speaker to be a little louder (someone already mentioned room boundaries, and I agree with them).
Be sure the channels are level matched, both to 75dB using your receiver. Run the receiver's internal pink noise test tone with an SPL meter to do this. Put the meter at ear level, facing upwards at the listening position.
If you already know how to do this and have done it, ignore this post.
Just to put your mind at ease some, there are some CD's that mix the vocals off to the side a little. A good example are the songs by Ray Charles on Jim Salk's Demo CD. I've noticed on three different systems that Ray's voice is a little off to the left, especially on the songs he sings duet with someone else. Just food for thought.
Anyway, start simple by checking speaker levels and distances and what not, then move along to a possible cable or interconnect failure. Hopefully you won't have to progress further than that.
