Hot water tank AO Smith ProMAx Model GNR 40 100 not igniting

[paulcoats]

Water tank was off after returning from vacation.  Got pilot lit after a couple tries and it worked OK for a week or so, then had to relight pilot again.  Did this a few times, pilot would light, gas burner would light up and we would get hot water for a day or two.  Then pilot would be out again.   The water was luke warm so I knew the tank was out again.  I got the pilot lit, it stayed lit when I let reset button go, turned the gas control knob to on, and turned up the temperature dial to Hot and heard the gas start running like it was flowing good.  But the tank did not light right away.  I could see the pilot was still on so I waited maybe 5 seconds then started to get worried gas was building up. Turned the temperature knob back down to vacation and heard the gas stop.  Waited a minute and could see pilot still lit and turned the temperature knob up again and heard the gas flowing well again.  Waited another 5 seconds and then got worried again.  Turned control know off and temperature down to vacation.

Any suggestions?  Am I just not waiting long enough for the burner to ignite?  Or do I need to start troubleshooting something or checking components, maybe take the burner assembly out and check for any debris ?