Check to see if it is wired properly. Often there is a wiring problem. If it is wired correctly, then there may be a manufacturer defect. Call customer service.
Likely, the problem is associated with a burned out heating element. This may be easily verified by making a resistance measurement across the element.
One should begin by going to the product guide and looking in the chapter that covers troubleshooting. You can also look on the internet for information on troubleshooting your specific model of hot water heater.
