Here are some tips that might be helpful to yield fluffy muffins - Use a leavening agent - In these sweet potato oatmeal muffins, we are using baking powder as the main raising agent. Hence, do not omit this ingredient as it is key in creating fluffy muffins ! Avoid overmixing - Overmixing cause gluten in the flour to develop, which is something you don't want to happen as the muffins will turn out to be dense. Hence, I recommend mixing it by hand, not a blender or mixer. And do mix the batter until just combined. Make sure oven is preheat - The initial surge of heat will push the baking powder into action, allowing the muffins to rise perfectly. Do not open the oven door frequently - Opening the oven door frequently will cause fluctuations in the heat, hence might cause the rising muffins to collapse. I do not recommend checking on them before the 20 minutes mark.