This is a common complaint from many gluten-free baking beginners. Gluten-free baking is often more science than art, and while environmental factors like humidity or even altitude can have an impact on your baked goods, your measuring accuracy (or lack thereof) is most likely the culprit. Pay close attention to measuring your ingredients, both dry and wet, as accurately as you can.
Also, the temperature of the ingredients could be a factor. Remember that many gluten-free recipes call for certain ingredients like milk, butter, eggs, and water to be at room temperature before incorporating them into the recipe. If you begin with cold ingredients straight from the refrigerator, you could have different results. Even ovens and their temperature calibrations can have an effect. Get to know your oven well so you can better predict how it will perform for you in making gluten-free baked goods.