A sheared key can sometime bind the flywheel on the taper. Try tightening the forcing screw until you can't tighten it anymore then using a 2# brass hammer give the forcing screw a good wallop. Then check to see if the forcing screw loosen any. If it did re-tighten and smack it again. It might take several times of re-tightening and smacking. As long as the forcing is loosening each time the flywheel is moving.
But be carefully and check crankshaft stub and make sure it is not the one getting smashed. So far I only had one flywheel I couldn't remove but someone had installed it using an impact gun.