Overheating, fan not coming in

We have changed the water pump and the thermostat, flushed the hose through the engine all good, however the top radiator pipe is hot and the bottom is cold ! The fan doesn’t kick in at all and it boils out the bottle, the fan works fine when we over ride with the computer., why ??? Anyone else had this problem ? We have at least got heat gh from the heater inside as we didn’t before we changed everything.