One speaker louder

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...

Wwshull tells it like it is

I swapped them, but no difference. You can tell they are the same speaker, but one has an airy quality, the other is more mellow.

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?