Gas fireplace flame keeps going off after 20 seconds.?

As winter season is coming, I maintained gas fireplace and found something wrong. I turn main gas valve on, switch nob from 'off' to 'pilot' and press & hold ignite on. Small light on bottom burner comes on. Turn the nob to 'on' position, and turn the main switch on. Flame lasts about 25 seconds and goes off immediately. Totally goes off. Even small light on bottom burner goes off. I need to start it again from the beginning over and over. What did I wrong? Please advise.