The most likely problem is that your baking powder is ineffective either because it got damp or is too old. But there are any number of things which can be going wrong.
Let's start out with:
1) A detailed description of your technique, including type of muffin tin, size of muffins, preheating, temperature(s), cooling, how you mix, how long the batter sits before baking; anything else you can think of;
2 A detailed description of how things look and feel through every phase of the mixing and baking process; and,
3) Your basic recipe, if there is one.
Please get back to us,