How can i fix my hot water problem?

My economy7 timer box doesnt is not working properly. The make is horstmann. I normally have the hot water on continueous. But it now it only works with the boost mode twisted round for one hour. The water tank has 2 elements top & bottom. The unit seems to be working ok, the off peak light works and the boost light works and the clock ticks round. But i just doesnt heat the water without using the boost mode
Can anyone help me