First of all check that is indeed the turntable and not the mixer.If it is the table after all, check the connections between cartidge and arm..erm...make sure its clean too!!Check the connections to the mixer.......thats all i remember now....affliction probs can tell you more
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