I installed new unitrol gas valve in water heater. It lit up and worked great for one afternoon and evening. It would not release gas to burner. I have tried three new thermocouples. The pilot lights the thermocouple heats up. But when releasing pilot and turning to on the best I get is a second burp of gas then nothing

Are you sure you have the correct kind of gas? LP or Natural Gas are not interchangeable unless you change the jet.