The water has to be boil, ie. not just hot but rolling boil. Lukewarm water may not ensure softer chapathis for longer period.
Do not add flour to the dough if the dough turns cold after kneading. It will harden the chapathi. Only add additional flour if required when the dough is still warm.
Knead sufficiently till it becomes non-sticky.
Cook the bailed chapathis immediately.
Do not keep the bailed chapathis on top of each other. They will stick.
Make sure to nicely clean the bailed chapathis of any residual flour. This will keep your chapathis without any dark spots. The flour will leave a black residue on the chapathi if not dusted properly.
For this chapathi, there is no need for putting any oil or butter. They are soft in itself.