Are you over mixing them? Or maybe not putting enough batter in each muffin cup? Fill each one 3/4 of the way. When you make muffins, mix your dry ingr. in one bowl, and your wet in a seperate one. When you mix the two together, mix just untill the dry ingr. are moist. Small lumps of flour are ok. Also, check your oven temp. If it's to low, they won't rise as much. Good luck.