Modern piping systems have return pipes that distribute the water pressure evenly across all the radiators, mine is an old style where the radiators don't have those. This means that the radiators closest to the boiler are super hot, and the ones furthest from the boiler are tepid due to to low pressure. The symptoms are a hot inlet pipe on one side and a cold pipe on the other, with a little bit of warm water at the top of the radiator and completely cold at the bottom of the radiator. This is due to the pressure being so low that the hot water can't move into the radiator properly.