Heating problem between upstairs and downstairs radiator's?

I have a alpha eco 240 ocean.. My upstairs radiators are hot but downstairs are cold! I was told it was the filter but can't find it on the boiler and got told it could be my pump.. Can anyone give me an insight to what the problem may be. I have purchased another pump to see if this will rectify the problem, but if it doesn't I'm back to square one