Here are 14 common reasons for flat cookies: 1.Inadequately softened butter. 2.An oven set at too high a temperature. 3.Using a hot cookie sheet or one that's been greased. 4.Incorrect measurements of ingredients. 5.Using the wrong type of fat in the recipe. 6.Neglecting to chill the cookie dough. 7.Undermixing or overmixing the batter. 8.Rolling out the cookie dough too thin. 9.Utilizing expired ingredients, such as baking soda. 10.Insufficient mix-ins in the cookie dough. 11.Baking too many cookies simultaneously. 12.Employing low-quality ingredients. 13.The recipe you're following may simply not be effective.