One speaker louder

You can tell they are the same speaker, but one has an airy quality, the other is more mellow

A dead or half dead tweeter perhaps?

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.

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...