Printer jamb fix – heat up extruder to normal temp then lower to around 90 degrees. Give override to extruder to allow retracting movement. See info taken from GCODE documentation at reprap.org. You may have to “Help” the extuder pull the filament up and out of the hotend.
M302: Allow cold extrudes This tells the printer to allow movement of the extruder motor, when the hotend is not at printing temperature