Assuming you checked your source/amp then Room Placement can play a small role. If you are listening to CD's then quite often the balance is such that sounds are divided in the soundstage - some left some right.
Assuming that you have indeed confirmed that it's the speaker and not something else, that is, you've swapped them left for right and the sound follows the speaker, then there are only a couple of possibilities:
1. Damaged speaker - a failure in the crossover network could easily cause what you describe, but occasionaly a driver failure will cause it as well.
2. Manufacturing tolerances - just because they're consecutive serial numbers or a supposedly "matched pair" doesn't mean they were perfect to begin with. Speakers are made by people, and people make mistakes.
are you using tubes?????? If so its possible one channel is dying slowly.......Check your speaker cable fittings...Try switching speakers ( leave everything else the same )..Just a couple things..Hope they help...........
I swapped them, but there is a real difference. You can still tell they are the same speaker, but one is quieter and less open.
Bad news. Next step (if new or recently purchased) is to take grills off and check no packing material is in the way - also check to see if a driver is completely dead and then contact the seller. Otherwise - you might be able to open them up and check connections and contacts and the crossover. What model are these?
Any chance you overdrived them - even briefly by mistake?
Finally you are looking at a repair or replacement driver...