The short answer
Hard rotis usually mean the dough was too stiff, it was not rested, too much dry flour was used while rolling, or the tawa was not hot enough, so the roti cooked slowly and dried out.
Fixes
- Add more water for a softer dough.
- Rest the dough at least 20 minutes.
- Dust lightly while rolling.
- Heat the tawa well before the first roti.
- Wrap rotis in a cloth as soon as they are done.