If your macaron shells don't rise in the oven, a few issues could be at hand. An under-beaten meringue won't contain the structure and air bubbles that give macarons their signature pied (frilly feet). Also, if your oven isn't hot enough, a lack of steam escaping from the batter will prevent rise. You also could've overmixed the batter, knocking out too much of the air in the meringue. While you need to mix enough to create a smooth, shiny batter, too many strokes of the spatula will lead to a deflated one. And macaron shells that wrinkle and fall flat when baking.