There's an adjuster screw on the top of the box, inside a nut. You loosen the nut slightly, and with an Allen wrench, tighten the screw, no more than 1/8 of a turn. Then retighten the nut without tightening the screw. It's a 2 hand job.
It's there to compensate for wear. Any more than that and you'll need to rebuild.
