The lower radiator pipe may be cold due to various reasons, particularly the following:
radiator soiling;
thermostat jamming;
ingress of air into the system;
malfunction of the water pump.
In order to define a defect cause, apply for help to specialists. Remember: the radiator lower pipe not always should be hot. At low environmental temperature, when the engine is not warmed up and cooling liquid circulates through the small circle, it will stay cold. It is normal.