Here are some tips to make soft rava idli. Don't skip Eno or baking soda. That will make the suji idlis fluffy and prevent them from becoming hard. Just ensure that it is fresh/active and within its shelf period. Another thing to remember is to add it to the batter just before you pour it into the steamer. The batter will immediately start bubbling, and become frothy. Mix it lightly till the batter appears airy and frothy. Important note - If you rest the batter for a long time after adding Eno, then the idlis will not become fluffy. They will turn out hard. The other reason could be that the batter is thin and too watery. This could happen if the curd is watery or if you have added more vegetables. Make sure that the batter has medium-thick consistency like normal idli batter. If they are sticky, it means that you added more water to the batter than required. It also could be because you didn't roast the rava properly. If you add more than the required amount of leavening agent (Eno), it could make the idlis crumbly. If they become very dry, then it means that the water in the batter was less than required, and/or you let it steam longer.