* Specific Trainers: Each Battle Frontier facility has a set of trainers you need to defeat to progress. You might be blocked by a specific trainer who you haven't beaten yet.
* Battle Points: To access certain facilities, you need to accumulate a certain number of Battle Points. You might be blocked from entering a facility because you haven't earned enough Battle Points.
* Missing Key Items: Some facilities might require specific key items to unlock them. For example, you need the Frontier Pass to enter the Battle Tower.
* Specific Story Requirements: There are some elements of the Battle Frontier that are tied to progressing through the main story. You might be blocked from accessing certain areas until you have progressed further in the game.
To figure out specifically what's blocking you:
1. Check the Battle Frontier Map: Look at the map and see which facilities are marked as accessible and which are locked.
2. Talk to the Frontier Brains: Talk to the Frontier Brains at each facility to see if they have any hints about what you need to do to proceed.
3. Check the Requirements: Read the descriptions of each facility to see if there are specific Battle Point or key item requirements.
4. Consider Story Progress: If you're at a specific point in the main story, you might need to progress further before you can unlock new areas of the Battle Frontier.
Let me know if you have any other questions about Pokémon Emerald!