The first thing you should do is turn the heating on, all the radiators that heat up switch off. If you have radiators upstairs and downstairs that is not working, start by turning the upstairs one off as well. The downstairs one should start to heat up. Once hot leave for 30 minutes before introducing the upstairs radiators. You should find the upstairs ones start to heat up. Once hot, introduce the rest of the radiators you switched off. If the problem still persists then you have a circulation problem. The system may need flushing out. It's best to let a local boiler engineer check your system for you.