There are typically 3 reasons why your mug cake isn't rising: 1) Leavening Agents: If the recipe calls for baking powder or baking soda, make sure they are fresh and have not expired (and don't leave them out!). If they are too old or if you haven't used enough baking powder or baking soda, the cake will not rise as it should. If your recipe doesn't call for baking powder or baking soda, make sure to use self-rising flour instead. (Note: this Biscoff cookie cake calls for baking powder.) 2) Microwave Power: If the microwave power is too high, the cake may cook too quickly and not have enough time to rise. 3) Overmixing / Thick Batter: Overmixing the batter could make it too thick, and it might be too heavy for the leavening agents to lift, causing the cake to not rise properly.