Its usually one or a mixture of three things, all of which have already been mentioned - sludge (rad off), air (bleed) or sticky thermostat (take control off and free the valve pin). I did however once have something different which foxed me for a while - some corrosion had built up just inside where the bleed valve sits - I could take the bleed valve out and nothing happened. Eventually I stuck some wire down the hole and that broke the "seal" and suddenly the air started to bleed out. Corrosion builds up more where there's air so rads that are susceptable to air (usually first one after the pump) will corrode more than the others. I always use Fernox and top up every few years to keep that at bay.
