There's nothing more disappointing than a tray of muffins that look puffed and rounded when the oven timer rings but sink in the middle a few minutes later. Here are a few reasons why that might be happening. You haven't cooked them long enough. Oven temperatures vary considerably. One way to check the actual temperature of your oven is to have a stand-alone oven thermometer in your oven. Use a digital thermometer inserted in the center of a muffin. Put it right to the bottom of the pan, then slowly pull it back toward the center. It should read 200 to 205°F. (93–96°C) If the temperature is under 200°F. they won't be fully cooked. Your leavening agents (baking powder and baking soda) are past their prime. If they've been in your cupboard for a long time, they may have expired. They usually last about six months, so if you're having trouble with muffins and quick bread sinking, they may be the culprit. You measured incorrectly. Use dry measuring cups for dry ingredients and liquid measuring cups for liquid ingredients. Use the "scoop and level" method for measuring flour. You substituted ingredients or added extra ingredients. It's tempting to add a little more of this or a little less of that, but gluten-free baking can be unforgiving if you don't follow the recipe.