bendix is a good start.
you said the starter is spinning you didn't mention a grinding or squealing noise.
so your bendix isn't engaging the ring gear. this is caused by dirt, binding/malfunctioniing bendix, or not enough speed from starter(low battery, faulty wiring, or bad brushes/ armature). usually if the spring breaks the bendix won't retract.
first thing as said ^^ clean the crude out of the bendix. make sure the bendix is free. if it still won't engage take the starter off, and hook up jumper cables to a good 12 volt battery(car battery is fine) hook the positive on the nut(not the stud) and bump the body of the starter with the ground. if this throws your bendix you can bet its a battery or wiring issue. if not you probably have bad brushes or armature. your dealer can rebuild your starter, or i could tell you how.
let us know how it turns out.