what prevented the boys from being rescued?

Many things prevented the boys from being rescued. One of the most telling episodes (you might be referring to this one) is when Ralph sees a ship in the distance. Ralph, Simon and Piggy run up the mountain knowing what they are going to see. Jack and his hunters were responsible for keeping a signal fire going. The promise of fun and hunting made them long forget this important duty. When Ralph makes it up he sees the ship fade in the distance and just embers from the fire. When he sees Jack ralph says, "There was a ship. Out there. You said you'd keep the fire going and you let it .." THe reason they can't get rescued is because most of the boys simply are not interested in rescue. THey live for the moment and no more.