Before you pull the starter, make sure your battery is fully charged. You need a strong battery to:
1. Close the solenoid
2. Spin the starter
3. Engage the bendix
4. Provide voltage to the coil.
As the battery gets weaker, the first thing to fail is your spark. If the battery is almost totally dead, all you will hear is the solenoid clicking. Your battery could have just enough current to spin the starter but not enough to engage the bendix.
If that's not the problem, it could be a dirty shaft (clean it w/ brake cleaner & don't lube it) of a bad ring gear (loose or missing teeth)