One reason might be that the dough was too dense to begin with. Another possibility is that the oven wasn't hot enough. The biscuits need a blast of heat in order to rise, so if the oven wasn't hot enough, they won't rise as much as they should. Finally, it's possible that the biscuits were overcooked. If they're overcooked, they'll be dry and crumbly, and won't have the same light, fluffy texture that's necessary for rising.