Is the problem with just one of the four speakers? If so, and if the speakers are okay, there's some kind of wiring issue, I'd suspect. Because whichever channel is driving that speaker, it's driving another just fine. That seems to indicate the amp is working correctly (it cannot send out a different output to one speaker than the other - - it's not that smart.)
If it's the left or right, check and see if you have a balance control for zone 2. Maybe it's out of whack.