Though Charlie reached the third level once, why could he not reach there again?

The story in this chapter can be understood on various levels.  

On one level, we might say that Charley actually reached the 'third level'. Your friend Chinmay has correctly answered the question, if you consider the story on this level.

However, if we consider the part of Charley reaching the 'third level' a figment of his imagination and a waking-dream wish fulfillment, we can say that Sam could not reach the third level again because he hadn't been there in the first instance; he only imagined that he did.

@Chinmay Good attempt!! Keep up the good work of helping your friends!