There are a few reasons why your pork belly skin might not be crispy. One possibility is that you didn't cook it long enough. Pork belly skin needs to be cooked until it's crisp and golden brown. Another possibility is that the skin wasn't scored properly. If the skin isn't scored, the fat won't render out properly and you'll end up with tough, chewy skin. Finally, make sure you season the skin well in advance to remove any access moisture as damp or wet pork skin won't crisp up.