All my radiators are fitted with Satchwell Thermostatic valves. When the central heating is on all radiators except one are hot. This one radiator remains cold even with the valve at its highest setting. The pipe leading into the bottom of the valve becomes hot but the output side remains cold. I've tried tapping the valve with a hammer and repeatedly turning the setting from high to low;but nothing seems to work! Does anybody have any 'trick of the trade' I can try before I have to pay a plumber to fix it?