There are a couple of reasons for hard rotis. First is the quality of flour. Second, if the dough is too tight, the rotis will be hard. They will not puff up and won't be soft. Thirdly, if the rotis are rolled with too much of flour then also the rotis get hard upon cooling. Fourth, if the rotis are left in the open after roasting, they get hard. The steam inside the rotis help keep the chapatis soft.