radiator not heating

i have an open vented heating syst em (oil fired). the problem being i have 1 down stairs rad which is fed from pipe drop from above, gets cold every couple of weeks. I have tried to bleed air from it but it seems to have no air in it as water comes out. Iv had to loosen the valve on rad and drain some water out of the flow and return pipes this does the trick but only for a short while and then have to start all over again a week or two later.
any suggestions to solve this problem would be most welcome.

Try a search for Balancing.

Thanks for the suggestion, will try & keep you posted of any success.

Thanks GASMAN balanced system by closing return valves down a little on other rads hope it all works out, days ahead will soon tell. Info much appreciated.