hi, from vast experience and water all over the house I would say that you should check that your pump is working, as heat rises and upstairsrads would get hot but down stairs would only get warm . if you put your hand on the pump and get someone to switch the heating on you will feel the vibration if it runs. also there is usually a speed control on the pump, which you could turn to max. also bleed the down stairs rads with the pump and heating off and empty a couple of pints from each one in case there is an air lock somewhere.
alf.