Water not getting hot

Question

Hi i have a dishlex dx302 i have power on the active side of the heater however on the nuetral side which is switched by a relay on the control panel the relay is not pulling in to supply nuetral to the heater the relay tests fine but it is not getting current to the low voltage side to make it work i prosume there is a thermerstat input some where. Can you help if not do you know where i can get a circuit diagram.
Kind reg Kevin