My water heater pilot turns off after about an hour
A thermocouple makes 30 mv power by a temperature differential across the tip of the thermocouple which contains 2 dissimilar metals.
If the entire tip is hot,there is no temperature differential thus no power made or  not enough power will be made.A thousand millivolts = 1 volt,so a thermocouple's 30 mv
is about 1/33 of 1 volt,a AA battery has 1.5 volts which is 1,500 mv,as you can see the amount of power made is very small, consequently it does not take much to affect the ability to maintain the pilot light. Many think that putting the entire tip of the thermocouple into the pilot flame makes more power, exactly the opposite will happen.
A temperature differential must exist across the tip, in other words the base will be a cold junction  and the tip a hot junction and this will create a tiny bit of power (30mv),
If the entire tip is hot no power or insufficient power will be made to power the tiny electromagnet in the gas valve that holds open the pilot valve after we have depressed it's button and lit it then held it closed for a minute or so.