The most likely culprit for flat biscuits is over-mixing! Make sure you only mix your dough until just incorporated. You want to ensure your dough has enough lightness to trap the air from cooking to create the perfect mounds. Expired baking powder or baking soda could also be making your biscuits flat. Sometimes those boxes get pushed to the back of the cabinet and we forget how long they've been there. To test your baking powder, simply put a teaspoon or so in some hot or boiling water. If the mixture energetically bubbles, it's still good. If there is no reaction or if the baking powder just fizzles, it's time to throw it out! You can do the same experiment with baking soda, just use vinegar instead of water!