This could be due to a number of reasons. The first could be from over mixing the batter. Be sure to only mix the batter until the flour clumps are broken up. The second possibility could be from the oven temperature. These muffins are first baked at a high temperature (425 F) to bake the outside of the muffin and cause them to rise really well. Then temperature is then reduced to 350 F bake the inside of the muffin. If these muffins are only baked at 350 F, they won't rise as well.