In a regular car, this would be diagnosed as a malfunctioning Bendix gear. I assume this starter is the same. First, always make sure you have a fully charged battery before assuming this is the problem. The bendix gear is on the front of the starter and when the starter starts running,, centrifugal force makes the gear go forward and the starter then engages a large gear on the motor. The motor then turns and starts. So it sounds as the starter is running, but the bendix is not being activated.