Bruce,
Stepper motors don't like being switched on to high speed from rest. They must be accelerated "slowly" to their final speed. This is particularly important when they are connected to their final mechanical loads.

The field created in the stepper coils just "bounces" over the rotor without imparting any motion.

Accelerating it manually is often enough to jump start the thing, as you found out.

What was your drive circuit ? The best drive is usually a constant current one, because then the magnetic field is established as fast as possible, in response to demand.

Steve