There are a few reasons this could have happened: 1. Not measuring flour properly! I highly recommend using a kitchen scale, as this will help you achieve the most accurate results. Too much flour leads to the cookies not spreading, whereas too little flour will cause them to spread too much. 2. Baking soda that is old or inactive would cause your cookies to not spread. 3. Not chilling the dough! This dough needs to chill for at least 1 hour before baking.