Even though it might be an obvious question, do you have Strago in the party? The chest won't ever let you pass if he isn't in the party. Also, you need 22 pieces of coral, according to Djibriel:

Quote Originally Posted by Djibriel
In every chest, there's an equal 25% chance you find one of the following:

1 piece of Coral
2 pieces of Coral
3 pieces of Coral
5 pieces of Coral

The talking chest will let you pass on two conditions. One: you need to have Strago. If you don't have Strago in your party, you can feed him all you want,but won't accomplish anything, either for you or for the chest. He eats, like so many young people of our time, because he's unhappy. It's a destructive and endless cycle of finding consolation in the very weakness he despises. I yap. Two: You need to feed him 22 pieces of Coral IN ONE GO. If you feed him 18, he'll send you off for more, and you still need to feed him 22 pieces of Coral again.