How does the failure to express himself in Chapter 9 by the symbolic Simon relate to the conflicts of Jack, Piggy, and Ralph in Chapter...

In "Lord of the Flies," Simon represents the intuitive, spiritual part of man if perceived as an allegorical character.  It is Simon who is helpful to Ralph, assisting him with the shelter construction when the others run off; he is always kind to Piggy, even when others ridiucle him.  But, because there is something different about Simon, even Piggy dismisses him as "cracked" after a meeting. 

In Chapter 8, after heated discussions among the boys, Simon slips away and goes to his secret place in the jungle.  There he observes the hunters who kill a sow and leave the head:  "This head is for the beast.  It's a gift," Jack declares.  Then, although "what was real seemed illusive" the intuitive Simon recognizes what the beast truly is as he looks the beast in the eye and his right brain/intuitive side recognizes evil.

and his gaze was held by that ancient, inescapable recognition.  In Simon's right temple, a pulse began to beat on the brain.

Grasped by an epileptic seizure, Simon "hears" the Beast say to him,

'You knew, didn't you?  I'm part of you? Close, close, close!  I'm the reason why it's no go?  Why things are what they are?

After falling unconscious, in Chapter 9, Simon regains his strength and beings his return; along the way he bumps into the decaying carcass of the parachutist.  Repulsed by this, Simon nevertheless frees the carcass from its entaglement.  Realizing what the boys have perceived as the beast is harmless albeit horrible.  Simon hurries to tell the camp, but he staggers, as the goodness in the boys staggers in the face of the anarchy of Jack and the savagery of the hunters.  For, the spiritually unbridled forces of nature bring the beast to the boys as they become more and more removed from the order of their British society.

When the boys kill Simon, they kill the spiritual side of man with evil ruling them.  Yet, at the same time Golding suggests that this evil is inherent in man, a natural force as when Simon's body goes to the sea, there is the natural cleansing of Nature.  Then, in Chapter 10 as Ralph tells Piggy that Simon was murdered, the emotionally weaker Piggy does not want to accept this truth; instead, he seeks to rationalize, saying "it was...an accident."   Ralph also lacks Simon's insight as he admits to being frightened, but does not know why. He becomes further confused about the fire when he cannot remember its importance:

Ralph tried indignantly to remember.  There was something good about a fire.  Something overwhelmingly good. 

Soon, because anarchy rules, Jack's boys attack the camp, and in a demonstration of their force, they gain control of the fire, a symbol of power.  Also symbolic is the theft of Piggy's boken glasses--the rational insight--as "dark figures" attack him.