The starter bendix is bad.
Could be a number of reasons:
Weak battery
Bad connections
Worn brushes
Solenoid contacts burnt, oxidized or just plain wore out.
Bad starter motor bushings allowing the armature to drag on the pole shoes.
Bendix problems
If I remember correctly the starter drive needs the rpm of the pony motor start to kick out the drive gear so your hand cranking is not fast enough to kick it out.