There are many reasons gluten-free baked goods can turn out poorly. And by poorly, I mean crumble, sink, explode, and appear to be useful only as a doorstop.
Sure, there are lots of factors, however, you can simply ruin any baked good by using old baking powder. Yep. If your baking powder isn’t fresh enough, it won’t work properly, making the end result have the wrong texture and rise.