By nozzles do you mean just the brass pieces that screw in? If so those have little or nothing to do with heating. They are the parts that get hot, yes, but heating is done through the resistor in the hot end and monitored by a thermistor, also in the hot end. You should see 4 wires coming out of the heater block just above the brass nozzles. It's possible that while replacing the nozzles you damaged the thermistor and/or the wires running to it or the resistor.