What is the significance of Ralph deciding to call an assembly when he did?

Ralph sees Jack's master manipulation at work. Ralph has turned his shame of not being rescued around to make Ralph look bad. How did he do that? Through the humiliation of Piggy and an insincere apology, Jack is able to come out on top. Ralph feels his power slipping into an abyss. He has to do something to save what influence he has left so he calls a meeting to set things straight.