This is a good question. Ralph is upset when the boys let the fire go down, because he is the most responsible boy, and he assumes and is given leadership. More to the point, he wants to be rescued from the island. Hence, when the fire goes out, there is no signal fire, which means that no one will find them. Moreover, in chapter two the boys were reprimanded for not taking the fire seriously. So, everyone knows to maintain the fire. What makes the situation even worse is that Piggy sees a ship on the horizon. So, hope of rescue dwindled even more.
As for Jack, he could care less about the fire, because he had a different obsession. His obsession was to hunt the pig. So, even though it was the responsibility of the hunters to maintain the fire, they were busy hunting. When the fire went out, they were out hunting. However, they came back bloodied carrying a pig. They were successful in the hunt, but failed in maintaining the fire.