Here's what should have happened:
You beat the Elite Four, checked out Twinleaf and Sandgem Town(talk to everyone), and went to Snowpoint City to take the ship to the Battle Frontier. Once there, you will find Flint and Volkner. After talking to them, they challenge you and your rival(who should have met up with you) to battle.
Is anything other than that happened, your game is glitched.