What are some quotes that show how the conch lost its power in the middle of the novel or later in the book Lord of the Flies?

The conch loses its power when Jack and the hunters break off, leaving civilization for savagery.

The conch is symbolic.  When the boys first gather together, it is because Ralph blew the conch.  From that point on, it holds a sort of mystical quality.  Since it had the power to bring them together, they think that it can keep them together.  This is why they choose Ralph as leader, even though Jack is a more natural choice.  Ralph’s qualifications begin and end with holding and conch.

They boys use the conch to take turns when speaking, acknowledging its power and granting it further power.  The conch becomes symbolic of civilization, and control.  Whoever has the conch controls the conversation.

“…I’ll give the conch to the next person to speak. He can hold it when he’s speaking.”

“But–”

“Look–”

“And he won’t be interrupted: Except by me.” (Ch. 2)

However, not everyone on the island wants to be civilized.  From the beginning, Jack and his choir exhibit signs of savagery.  They are assigned as hunters, which means that they will kill.  Soon there are war dances, war chants, and even war paint.  As the hunters splinter off from the main group, their behavior gets more and more primitive.  Soon there are the hunters and the others.  Ralph no longer has the control he once had, and neither does the conch.

“I got the conch,” said Piggy indignantly. “You let me speak!”

“The conch doesn’t count on top of the mountain,” said Jack, “so you shut up.”

“I got the conch in my hand.” (Ch. 2)

Eventually, the conch has essentially no power, because there is no one left to follow Ralph.  His attempt to keep the boys civilized has failed.  Jack’s way is more enticing.  No one follows Ralph’s summons except the littlest kids.

Jack seems to exist outside the control of the conch, because he threatens Ralph’s authority.  When Jack talks, even if he doesn’t have it, the boys listen.  Jack’s way of ruling is much less democratic.

“Conch! Conch!” shouted Jack. “We don’t need the conch any more.  We know who ought to say things. What good did Simon do speaking, or Bill, or Walter? It’s time some people knew they’ve got to keep quiet and leave deciding things to the rest of us.” (Ch. 6)

Jack continues asserting that the conch doesn’t count.  He establishes himself on the opposite end of the island, on an outcropping of rock, and tells Ralph that the conch “doesn’t count at this end of the island” (Ch. 9).  Ralph maintains that it counts everywhere, but he is outnumbered.  By Chapter 10, the conch is described as “fragile.”

It is no coincidence that Piggy’s death coincides with the death of the conch.  Piggy has been the voice of reason.  He thinks like an adult, making him a link to civilization and society.  His death eliminates the last vestiges.

The rock struck Piggy a glancing blow from chin to knee; the conch exploded into a thousand white fragments and ceased to exist. (Ch. 10)

The conch is gone.  There is no order.  The conch’s power existed in the boys giving it power, and as its power diminished, so too did their control over one another diminish.  Soon, they were reduced to a savagery and disorder.

The conch’s symbolism is rampant throughout the book.  As the boys show less and less regard for Ralph, who represents order, they show less and less response to the conch.  By the time it is destroyed, Ralph has no power at all.