Did you know that baking powder has a shelf life? Unopened baking powder remains good indefinitely, but once you open a container of baking powder its potency starts to wane. The ingredient in the baking powder that would react with liquid in your recipe reacts instead with water vapor in moist kitchen air. You can slow this process by making certain your baking powder is tightly sealed when you aren't using it.
It's a good idea to test baking powder before using it in a recipe. Mix a bit of warm water into a small amount of baking powder. If you see bubbles of carbon dioxide form, then your baking powder is good. If no bubbles are formed or the reaction seems weak, it's time to replace your baking powder.