It could be that the radiators valves upstairs are wide open this will offer no resistance to the water flow so it will go the easiest way. You will need to shut down each radiator at one end (The opposite end to the radiator thermostat if fitted) this end will probably have a cap on rather than a knob take this off then open the valve slightly. You will probably need to open the ones farthest from the pump a little more.
This should then build up the pressure and the water should then go downstairs. Follow the same steps downstairs.
Hope this helps
Trackman