Hi all. I woke up this morning to no hot water. Brief investigation revealed the pilot light lit but wouldn't stay burning when I released the safety, so I presumed the thermocouple was bad. I got one today, installed it with no problems, fired it up, and it stayed lit. The main burner lit no problem when I turned it on, and I heard it running for 30-40 min. I called for hot water a while later and it was lukewarm. I went to check on the tank and found the pilot light out, and again it lights but won't stay lit.

I'm quite confused why the new part would seem to have fixed the problem and then failed again so suddenly. Any thoughts?