This situation has happened to me before. If the engine is extremely difficult to turn, the load on the starter armature will not allow the starter drive gear to disengage from the flywheel. If the engine doesn't start, or at least fire a cylinder occasionally and unload the gear, the spring in the solenoid will not be strong enough to yank the drive gear out of the flywheel, keeping the contact disc on the solenoid engaged. The first time it happened to me, I was working on an engine that had the pistons fitted way too tight, creating lots of drag on the starter. Once we got it to start and the pistons established a running clearance, the problem went away. Try turning the engine over with a wrench on the crankshaft center bolt, and I'll bet you find it takes LOTS of torque to make it move.
Jerry
