Sometimes one-way clutch bearings can get jammed and not disengage. On the other extreme they will slip if loaded with too much oil or if the starter shaft is highly polished. In this case, sanding the shaft with 400-800grit sandpaper will give the bearing something to grab onto and stop the slipping.
Big block engines usually have the one-way clutch bearing inside the engine and smallblocks have them externally. I would remove the one-way clutch bearing (what they really are called), clean/soak in acetone for 5-10 minutes and blow out with compressed air. Put about a half drop of oil inside the bearing where the needle rollers are and reassemble everything.