I am having problems with my 40 gallon natural gas water heater Model GVC-40-100 made by A O Smith. The heater was installed in 2006 and worked fine for the first three years. Now I can't seem to keep the pilot lite lit at all times. I've changed the burner assembly twice and the thermocoupler three times. Each time I change something it works for a couple of months and then starts acting up. Very frustrating. I now check the pilot lite every hour , every day to make sure it lit or if not I re-light it. No clogs, drafts, dirt, defective fittings, etc.
Please help!!

This is a common problem especially with AO Smith Heaters. I usually change the pilot assembly. I also have a special wand for my shop vac, provided by AO Smith, for cleaning the combustion chamber. When the access panel is off, vacuum out the combustion chamber and then use a compressor(or some shop vacs can blow air also)and blow air down through the hole in the middle of the floor of the combustion chamber. Then remove the black plastic screen from the front bottom of the heater and vacuum that section out as well. There is a plate in there that has very small holes that are very easily clogged with lint, animal hair, dust, etc.
If a very thorough cleaning like this and a new pilot assembly(which includes a thermocouple) doesn't fix this then a new gas control valve is probably needed. You would need to call AO Smith and they will most likely walk you through some diagnostic tests with a multimeter and possible a manometer.