Did you bench test it before you installed it?
The thought of it spinning the wrong way did cross my mind but I don't know if it's possible. I don't think it's possible to put it in and miss the sprocket either, not enough room for it to get out of the way. It may be possible for the chain to be off the sprocket though.I suppose that you could have missed engaging the starter output gear with the sprocket that turns the starter clutch.
When you bench tested it are you sure it was spinning in the correct direction? I guess it is possible to put the brushes in the wrong holes but as I remember they would only go in one way and the wires wouldn't let them be put in wrong.