I renewed a downstairs radiator and pump in an open system.The bottom rads get hot but the upstairs ones remain cold. The system has been fully drained down & filled again & yes the pump is fitted the right way round & is working fine.I have bled all the rads, but one will only just dribble water out but still no air comes out ? If i take the bleed screw out of the other rads upstairs the rads heat up but when the screw is put back in they go cold again. Please help !!