There could be a few reasons:
1. Temperature of the dough: if your dough is not warm enough while it’s resting, the yeast will not start to grow and expand the structure of the dough - keeping it light and fluffy.
2. Under kneading: if the dough was under kneaded, it won’t rise properly.
3. Expired yeast: yeast is a living thing, and dried yeast needs to be reactivated. If your yeast has expired (or is dead), it won’t produce gas or air bubbles, and so the dough won’t rise.