Look at the adjusting nut hooked to the clutch, if it is loose, missing or mis adjusted will cause the clutch not to operate properly. To be clear, push the clutch in while looking underneath the mower at the rear connecting rod that is moving when you press the clutch in, it moves a small lever that is hooked to the rear transmission, there is an adjustment nut and spring on it that must be adjusted correctly for the mechanism to work.
Good luck,
Dave, May 2009