Nephi's bow had broken, but the brothers also had bows. What was wrong with theirs? Verse 21 indicates that those bows had lost their springs. What does that mean? "Now Saxton Pope in his classical work called Hunting with the Bow and Arrow says the average bow is worth a hundred thousand shots. After that it loses it spring and you can't use it anymore. [Nephi], who seemed to be a very capable fellow, must have been using his bow for years. It says that their bows had lost their springs, and that would happen (Hugh Nibley, Teachings of the Book of Mormon, Semester 1, p.218).
The family had brought their bows with them, and they were bows which had seen use for years. Unfortunately for the family, they "loose their springs" just around the time that Nephi's bow breaks. This appears to have been the last of the functioning bows.