I had a chevy starter in a jetboat that I was sure was having a problem kicking the bendix out. It would just spin up fast and not engage. Turned out that the one way clutch in the bendix was shot - the gear was engaging but the shaft was free spinning inside the pinion gear. The clue was that if you held the key on long enough it would suddenly engage and turn the engine over a few times than spin up again.

So - are you sure the pinion is not engaging?