There could be a couple of reasons. First, check the expiration dates on your baking powder and baking soda. If they are at or near the expiration, get rid of them and buy fresh. Second, make sure that you don't overmix your biscuit dough. Just combine until all the buttermilk is mixed in. It will look dry and not coming together and that's exactly right. It will come together when you do your cut and stacks.