Radiator only luke warm

I have a pressurised heating system. One radiator only ever gets luke warm, no matter if I shut all other radiators off, and open both radiator valves fully. I have taken the radiator off, and flushed it with a hose pipe, during which the water freely flows through the radiator.. With the radiator off, when I open the inlet valve i.e. incoming hot water (into a bottle) the hot water flows freely under pressure as expected. When I open the outlet valve only a dribble comes out... I would have thought with it being a pressurised system water would be forced out of the outlet valve? Needless to say, when I replaced the radiator it filled up initially with hot water, but the outlet pipe only gets luke warm, and the radiator then cools again to luke warm... How can I resolve this?