Inside the valve head is a little brass pin that goes up and down to control the flow - the pin sometimes (quite often) gets stuck down in the closed position. It's easy to fix it.
Unscrew the collar on the bottom of the control head and lift the adjustable head from the valve body. Either tap the valve to release the pin or pull it up gently with a pair of pliers.
All the way up is fully on. You can leave the control head off for a while to check that the radiator is heating up.
Peccavi, March 2010
