How do I make soft chapattis?
Yogurt and milk might work, but those ingredients are not in authentic chapati.  When I hear the words "hard" and "rubbery" in reference to pan-fried bread, I'm almost certain that the problem is that they are cooking for too long on too low of heat. Increase the heat on your pan (it should be very hot) and cook the chapati very briefly on each side. This will still give them a nicely browned exterior, without turning them into sheets of rubber.