Lift up the plastic cap covering the end of the arm at the point it is secured to the shaft. Beneath this is a nut holding the arm to the rotating shaft. If the nut comes loose, then either the arm can flop loosely or the inner splines on the arm can wear down. To fix the latter, remove the arm and clean out the splines with a wire or small blade. Reinstall and tighten the nut. Use Loctite if it tends to come loose.