It could be that the downstairs radiators are starving the upstairs ones - there are two valves on the radiators, one which you normally regulate with a thermostatic valve( if fitted ) the other you could try closing half way on the downstairs radiators to restrict the flow through them to "push" the water upstairs
( you could also try opening up the "other" valves on the upstairs a bit more ) - another possibility is that there is a motorised /zone valve for the upstairs which might be faulty there is normally a manual overide lever to open them manually -
anyone else any ideas ?