Here is a good (but somewhat long) video on how the starter bending works.
YouTube clicky
Check the voltage at the starter. If it isn't getting 11+ volts, you may not be driving it fast enough to throw out (up) the bendix gear. If you know the starter is getting 12v and you hear the drive spinning, perhaps the bendix is stuck. If it works on the bench, you have 12v to it in the car, and it is seated flush on the block plate, I don't know why it's not engaging.
Measure how far your bendix comes out. Make sure you got a 9403 instead of a (potentially mis-labeled) shorter 9404.
Good starter dimension document