If you keep ending up with flat muffin tops that spread, chances are it's one of three things: 1. Ratios are off for flour, fat, and sugar; 2. The leavening agents (baking powder and baking soda) are old and need to be replaced; or 3. You are starting with an oven temp that is too low. A high, fast blast of heat will cause the muffin batter to rise more quickly for taller tops.