This happens if: - You didn't bake the muffins at high temperatures for the first 5 minutes. - You used gluten-free flour - as mentioned above, gluten-free flour won't rise much. You can add 1 teaspoon of apple cider vinegar to the recipe to boost the vegan gluten-free muffins' rise. - Your baking powder or baking soda is old - make sure you are using a fresh raising agent.