Not getting full heat from the radiators?

I have a Potterton Primera combi boiler. The radiators are not heating properly, they are barely warm (as if each of them has been turned down low) and the pipes are the same. Throughout each radiator the heat is constant (not cold at the bottom or top) & have bled them but that's fine. The temperature is turned up full, the thermostat is on, the pressure is working. Anyone any ideas what it could be please?

Thanks

Suzanne
January 2010