The official Pokémon website has announced that the Mythical Pokémon Mew will be redistributed in June’s Trainer Club Newsletter for anyone that wasn’t able to receive it back in February. I suspect this is because there were multiple cases of GameStop running out of codes before everyone got their chance to get their Mythical Pokémon; every store I went to had already run out of codes by the time I got there!
It is noted in the announcement that this is the same Mew that was distributed from GameStop three months ago, so anyone who received one back then will be unable to claim this Mew.
Make sure you opt in to receive the Trainer Club Newsletter before June to receive your code!