well what i think is a bit of trouble
seems like this is the first thing you need to assertain
probably the second thing
there's not really much more to say that mr gordo didnt cover
check the carts, maybe give the connectors a lick
swap the carts over, and see if it still does it.
if it still does, check the connections.... Unplug / Replug
try also swapping the turntables over, if still the same channel on the mixer is quieter (ie still the left deck after you swapped them over), then its most like a problem with the mixer (im assuming you're familiar with the gain controls). Try using your mixer on a mates system, if it still does it there, you fucked