Are you turning it over without the plug? I only ask because you could have one of two probs. First, your flywheel key may have broken, and the flywheel is spining on the crank. Now, the crank is spining with the flywheel, but if the spark plug is out (this is why I asked) there is no compression building up, so that means the starter and gears doesn't have to work as hard, and as a result the flywheel stays stationary. Either pull the flywheel and inspect the key and keyways, or put the stator cover on, remove the cam cover and set the piston on TDC. With the "T" mark on the flywheel lined up with the notch in the inspection hole, make sure the cam gear mark is lined up with the bump on the head.
2nd prob, your starter clutch is no good and slips at a certain point during it's rotation.