Using a rolling pin isn't advised for making bajra roti because the dough is very delicate. Without the addition of wheat flour (atta), no gluten is created. While that is good news for celiacs, it is not great for those of you who are used to using a rolling pin. To simplify the process, I suggest using a ziplock bag, two sheets of parchment, or even a large muslin cloth to enclose the dough, then use your hands to press it out into a round flatbread shape. If you are still having difficulty, try adding a bit of potato flour/potato starch or whole wheat flour (atta) to the dough to help make it a bit more pliable.