cold upstairs radiators

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 !!

I'm not exactly an expert but had a similar problem. Try balancing the manaul valve (the one that turns the heat on/off on the radiator) with the lockshield valve (other side) on the new radiator;this can be done by buying thermostats that clip to the pipe just below the valves on both sides of the rad.Open manual valve fully & allow to warm up. Take the cap off the lockshield valve & adjust with spanner until thermostat on that side is 11oC (20oF) lower on that side than on the manual valve side.My D.I.Y book recommends balancing all the radiators when a new pump is fitted.Good luck.