Cold radiator

I’ve recently replaced a radiator downstairs and serviced the valves . The system has had sentinel 800 and been drained several times and purged of air. It’s 10mm microbire. Downstairs rads are on drops and other three downstairs rads work fine. The rad refuses to get properly hot. If you close the valve one end and open the drain valve there is a good flow of water through the rad which starts to get hot but only along the top of the rad and the same closing that valve opening the other end produces a good flow of water but the rad does not retain heat and does not produce heat when both valves are open . It’s as if the circulation isn’t right ie the hot coming from one end is circulated and the hot draining from the other us coming from upstair as the system starts to drain Either way I can’t see how there can be a blockage as the water flow even by gravity is good. The pump is at max speed and the boiler is new professionally fitted. I’m stumped! Any advice much appreciated! Thank you