Hello, I’m not sure I can help here, but it seems it would be good to check out two things, power and water: First, let’s see if power is getting to the upper element. You should safely measure about 240 volts at the element terminals when the heater is first powered. Do you get that voltage? Next turn the upper thermostat down as low as it will go. That should switch power down to the lower element. Do you hear the thermostat click over and do you get power down at the lower element? One more thing… it’s rare, but sometimes elements are bad. Check continuity between the two element screws (there should be continuity) and also continuity between each screw and ground (there should be none). Of course, do this with power off and at least one wire disconnected from the element.
Next, let’s look at water. Assuming you have a water meter, do you see it standing still for minutes when everything in the house is turned off? That would be a sure sign there is no significant water leak. Now, it also would be good to perform a cross connection test. I understand that it worked before, but one of the main tenants of troubleshooting is to assume nothing. The test is simply done by shutting off water to the heater and opening a hot tap. If water flows a bit and then stops, you have no significant cross connection. That would be good! So, please do these tests and let us know what you find. 😎