Some radiators are warm, some are cold???

Although all radiators are turned on, some of them won't heat up! we have 4 radiators downstairs, two work, two don't. We have 5 radiators upstairs, 4 work, 1 doesn't. It doesn't seem to be just the ones that are furthest away from the boiler, 'cause the one upstairs that doesn't work is actually the closest to it! Any ideas?