There might be several reasons for this. Puris need a hard but smooth dough. If the dough is dry and crumbly, and not smooth, it can lead to hard puris, due to lack of sufficient moisture in the dough. Puris can also turn out hard if you have rolled them out too thin. Or if you have fried them in oil that was not at the correct temperature, and as a result fried them for longer in oil that was not hot enough. See recipe & notes for steps to follow and avoid this.