been sitting a while? that happens when the clutch plates dry up and get sticky.. you need to pull the clutch out of it. you'll probably have to get a screw driver to pop each fiber off the steel plate it's stuck too.. then give each plate a quick soak in oil and toss it back together.. it's pretty common.. make sure to inspect each fiber and steel while your taking it apart. sometimes a cracked plate can cause the same condition.