What???? How dare you hijack a hijacked thread and return to the very question asked in the first place??!! ;)
" Is it possible the battery is low, and not spinning the starter fast enough to engage the drive? "
Yes, that is a possibility.
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.
The more current you use to spin the starter, the less you have for the ignition.
So the battery could be weak enough that it will spin the starter motor but not engage the bendix.