I've been making chapatis for a while now, and conventional wisdom says that getting the dough well knead is the key to soft chapatis since it forms the gluten strands in the dough and makes it tough. I very recently chanced upon the real method to make soft pliable dough that also results in chapatis that balloon even under sub-optimal rolling.
The key is to knead the dough in salty water. Take some salt (to taste) and mix it in some water (at room temperature). When the salt has completely dissolved in the water, use that water to knead your dough. You will find it much easier to knead your dough, and it will result in a soft pliable dough that is somewhat forgiving of mistakes (or chapati got stuck to the rolling pin and folded over itself) while rolling.
I additionally add some turmeric and red chilli powder to my dough for enhanced taste and colour! :)