Well it looks like patience is required when watching the motors turn
According to the markings on the motor, they are rated as 1 RPH - that's revolutions per HOUR! No wonder I couldn't see them turning.
I took erick's idea and stuck a small piece of copper wire (no toothpicks) to each motor gear. After 15 minutes, the movement is obvious.
I'm going to give everything a good clean, apply some grease and reassemble.