I guess that it depends on how your central heating system is designed, but when I had my system installed a few years ago, I seem to remember the engineer saying something about making sure at least one radiator was "on" when the pump was running. It could be worth checking that - additionally, if you have thermostatic valves on radiator should have a valve that is always on.
Failing that, it could be a blockage - I'm not a plumber, but I believe that you can put chemicals in it to prevent scale build up / corrosion etc.
Hope this helps!