how do i get Creda night storage heaters to come on?

I have just moved into a property which has got 2 creda night storage heaters. I have adjusted the controls( input and output) to regulate the heat. and switched them on at the mains. but they do not come on. I have had them switched on continuously for 4 days. but they have not come on they are still as cold as the day i moved in 2 months ago.... If anyone can help i would be grateful only winter is coming on now. I need the heat.