Two things come to mind:

Perhaps the thermostat needs replaced (not sure that is what you call
it). Its the thing that kicks on and off when the temperature hits a
certain temperature and allows the radiator fluid to circulate.

The other is whether there is air trapped in the radiator lines. Part
of the procedure to change your radiator fluid is to open a certain
screw or bolt or something and then wait until no more bubbles come
out. Its close to the thermostat as I recall.