A good cupcake will rise for a light, airy and domed result. But there are factors that can cause this not to happen, the most common of which can be the ingredients you're using. Expired baking powder (anything that's been open for more than a year) won't work. Mix a little hot water with your baking powder and, if it doesn't bubble, it should be replaced. Expired baking soda should be fine for 6 months after opening. Test it by mixing it with water and vinegar (because, unlike baking powder, it's activated by acid and not just water) and checking for bubbles.