Heat Upstairs But not Downstairs?

I have 6 radiators in my house, 4 upstairs and 2 downstairs. My upstairs radiators are fine but I have a problem with the downstairs radiators, in particular the hall radiator. I have renewed the two valves (inlet and outlet) and flushed it through but still cannot get any heat into the radiator. My central heating system works off a Glow Worm Swift Flow Combi Boiler. All heated Tap water in the house (both upstairs and downstairs) is working well. Can you help with this problem please?