Amount of baking powder in a muffin recipe
1 tsp of baking powder and 1/2 tsp of baking soda to 2 cups dry ingredients is actually more baking powder and soda than I found in the first 10 random recipes I checked from Food Network and America's Test Kitchen. That suggests to me that the recipes are fine. Could it be that your leaveners are losing potency?
You can check them.
From Chemistry.About.com:

How to Test Baking Powder
Baking powder is activated by a combination of heat and moisture. Test baking powder by mixing 1 teaspoon of baking powder with 1/3 cup hot water. If the baking powder is fresh, the mixture should produce lots of bubbles. Be sure to use warm or hot water; cold water will not work for this test.
How to Test Baking Soda
Baking soda is meant to produce bubbles when mixed with an acidic ingredient. Check baking soda by dripping a few drops of vinegar or lemon juice onto a small amount (1/4 teaspoon) of baking soda. The baking soda should bubble vigorously. If you don't see a lot of bubbles, it's time to replace your baking soda.