I was going to suggest slippage but not familiar with the robofocus/focusmax, I decided to keep my trap shut
If the robofocus is a stepper motor and it's an open loop system(no position feedback), it is possible with heavy load or excessive friction etc that the motor is missing steps. It will get the step pulse and will sound like it has moved, but the motor lacks the torque to make the move and springs back to the last step.
This is a common issue with CNC machines that use steppers(and one reason servo's are preferred, at a cost) and pretty easy to test with a dial gauge.