Jack allows the fire to go out in "The Lord of the Flies" because he is distracted by killing a pig. Just before he kills the pig, Jack paints his face with red and white clay. The clay makes Jack feel like a "stranger," and it makes the other boys obey him and follow him to hunt, even though it means they're leaving the fire untended.
The fire was built as a signal for help, and the other boys on the island realize that it has gone out just after they spot a ship near the island. They are disappointed with Jack and his friends for abandoning the fire. Jack becomes upset with Piggy for criticizing his irresponsibility, and he punches Piggy in the face. During the scuffle, Piggy's glasses break, and Ralph uses the broken lens to light a new fire.
Once the fire has been lit again, the boys begin to roast the pig. However, Ralph is so upset that Jack let the fire go out that he refuses to eat the pig. Eventually, Jack admits that he was wrong to let the fire go out, but he only apologizes to Ralph. He doesn't apologize to Piggy.
The scene in which Jack lets the fire go out is one of the most important scenes in the book. It shows the boys' conflict between being responsible and wild, and it contains symbolic imagery of power and weakness.