My banana Muffins won't rise like I want - Why?
Did you preheat the Oven?

At our facility we preheat to 400F then lower to 360F just as soon as we close the door. Why? Muffins do not rise enough (we feel) in a warm oven. The time the oven door is open also cools the oven itself 30-40 degrees. So we preheat, then it's at the right temp as soon as the door is closed. 

We try to cook fast in a hot oven rather than slow in a warm oven. You have to achieve the chemical reaction part rather quickly or it fizzles out. 

That is only one of many reasons for small round tops, as you're finding out. Here is another article that may also help the train of thought.