Why won’t my tank heat already changed both element?

thombomb on a dual element residential water heater, both elements do not heat at the same time. Starting with a tank of cold water, the upper thermostat will energize the upper element. When the water in the top half of the tank is hot the upper thermostat will shut the power off of the upper element and send it to the lower thermostat and element. That means that you will have power to the top only at first. When hot water is used, cold water enters the bottom of the tank at the same time. This will cool the lower thermostat and the lower element will heat. Replace the thermostat as well since this could be the issue.

Also, you have to make sure that the tank is full. The elements burn out really quick if the tank is not full. Had this happen myself on a brand new tank. Now whenever I change the heating elements, I turn the breakers off even when done. Then Open all the hot water taps in the house and run the water for a while. That gets the air out of the tank and ensures it is filled. Then I turn the hot water tank breakers on again and let the tank Do its thing. So, check your top element for continuity.