The main reason the Pita won't puff up is that the baking temperature or your oven is not hot enough. If in a stovetop make sure by drizzling few drops of water on the griddle if it sizzles then the temperature is right. You can also press the pita bread with a clean towel as it's getting cooked. Still, if the pita doesn't puff up it's absolutely nothing to worry about. They still are edible bread without a pita pocket.