Why won't my Charizard Mega Evolve when I press the button?

Question

I am playing Omega Ruby and have given my Charizard a Charizardite X. Despite this, when I go into the battle, I am unable to click the mega evolution button. It appears on the screen, but if I press the mega evolution button, and do a move. The Charizard will not mega evolve. If it matters at all, I have not beaten the Elite Four yet.

Comments

Do other Pokemon not mega evolve or just Charizard?

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just charizard

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Do you have a key stone and an appropriate mega stone?

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Umm...  The Charizard in question is holding a "Charizardite X".  As for the key stone, I have made it all the way to the Elite Four, so I have made it far past the latios/latias point where you obtain it.

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Have you already Mega Evolved in the battle ur on?

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The Mega Evolution “fades” when you check the summary of a move.

Answer 1

Okay, after some thought and experimenting, I have deemed the cause to be the mega evolution of a second Pokemon during the same battle. Example: Mega Evolving a Lucario but then not being able to Mega Evolve a Sceptile when the Lucario faints in the same Pokemon battle. thanks for helping all.