Try turning off all of your upstairs radiators and make sure both valves on all of your downstairs radiators are fully open, then run your heating. The downstairs rads should start to get hot if they don't there will be some sort of blockage in the pipe work or valve not opening. once they are hot downstairs turn back on your upstairs rads but just enough until they start to get hot (ie half a turn at a time).
This should resolve the issue temporary but you may need a blockage removing and / or a system power flush.
Hope this helps!
Super Dan the gas man, December 2015