My gas fireplace is not lighting, even after replacing thermocouple, what next?

A few weeks ago, I asked what I needed to do with my fireplace if I can get the pilot to ignite, but I can't keep it lit. Changing the thermocouple was the overwhelming answer. I went through the manufacturer and got the exact thermocouple and replaced it. Only to have it doing the exact same thing. I can hold the on in, ignite it, and it will stay lit as long as I hold it in, but as soon as I release to go to 'On' it goes out. what else?