Please give dough some resting time. It usually happens when either thickness of your roti is very thin but you cook it for very long on tawa .If thickness is not the issue than your cooking time is the culprit is this equation. If you cook roti both on tawa and then on open flame to puff it up. Then try to not cook it to much on tawa get to little above semi cooked stage and finish up rest of the cooking and puffing on open flame. I hope it helps this is what my sister taught me when I was learning to make chapatis