Extruder Motor Won't Move
There have been many posts regarding non-operable extruder motors. Possible causes are listed below. If you discover a cause that isn't listed, please add it (or ask the Rigidbot Google+ community to add it if you don't know how.) Put a dot on the end of the motor shaft to ensure that the motor is really not moving.
1) Firmware minimum extruder temperature. TRY THIS FIRST! The extruder must be heated to at least 170C or you must send a M302 command before firmware will allow it to move.
2) Defective motor. Plug the motor cable into X or Y axis and try to move with manual commands. TURN OFF PRINTER before connecting or disconnecting cables.
3) Defective extruder cable. Test for continuity using a multimeter.
4) Fried stepper driver. Half of the driver can fry causing the motor to behave as if one coil is disconnected. See: Setting_the_second_extruder_port_as_default_(Switching_the_2_extruders)