In order for a chemical leavening agent to release the optimal quantity of carbon dioxide gas into the dough or batter, the acid and alkali must be in proper proportion, as it is in the case of baking powder. When you substitute buttermilk for sweet milk, you incorporate extra acid into the batter or dough, which upsets the proper proportion of acid to alkali that was worked out in the original recipe. That extra acid will reduce the amount of carbon dioxide generated, thereby inhibiting the leavening process. To counteract the hyperacidity, the cook needs only to substitute baking soda (an alkali) for some or all of the baking powder in order to maintain the necessary quantity of alkali.
For each cup of buttermilk that you use in place of sweet milk, reduce by 2 teaspoons the amount of baking powder originally called for in the recipe and replace it with half teaspoon of baking soda.