Could be the circulation pump or thermostat, for which you need an electrician, or (as happened to me this year), simply an airlock. Plumber told me to turn off all rads. Then turn on rad nearest the boiler (the first in the lineup). Bleed the rad if necessary, and when that rad heats up fully, turn it off and turn the next one on. When I had turned on and off all the rads in the house, finally turn them all on together. Was a right pain, and took ages, but it worked!
Alice M, October 2009