cold radiator?

hi i ve read lots of your answers and followed your fault findindig tips which have got me to the point where i am, which is took rad off flushed out with warm water,opened non trv valve to see if hot water came through and it did,pin on trv valve seems to move free enough up and down,replaced rad opened non trv valve end and rad was hot for 15 mins then went cold again,both pipes to rad are cold any suggestions