Downstairs radiator won't come on?

One of my downstairs radiators wont come on. The flow and return valves at either end are fully open, both have flow & are not blocked & no air in system. I have taken radiator off & flushed it through with hosepipe, also turned every radiator off in house and still won't work. If I drain from return valve on Lhs hot water starts to fill radiator from flow but then radiator cools down again. Any ideas ??