Hot water circulator?

The hot water circulator in my Johnson & Starley warm air unit is not working properly as it has in the past. The unit heats the water and provides about a third of a tank of hot water in the top of the tank and then it stops. The flow pipe is very hot but the return pipe remains cold. There appears to be no circulation. I have drained off some water from the drain cock adjacent the circulator in case there is an airlock and I have checked that there is no blockage.