o Is the extruder stepper holding torque (to check if it's actually powered) ? I think it should be holding torque at all times when the machine is running

o The extruder could refuse to move if the temperature of the nozzle isn't high enough. Could you verify that the temperature reading is good and that the nozzle heats properly? A very common failure mode of Thing-o-matics is that the heating resistors on the extruder burn out.