First recommended thing for batter fermentation is to mix the batter using ur hands..Mix the dal and rice batter thoroughly.. Uneven mixing causes failures. Mixing them by hand works the best! A warm temperature is essential for fermentation. 25 to 28 °C (77 - 82.4 °F) will be ideal. If the weather is cold, keep some warm water in a large bowl and immerse the vessel containing the dough. To help maintain the temperature of the warm water bath, cover it with a blanket, or better, use a regulated heater, like a heating pad or aquarium heater. Even easier, turn on the light in the oven, and use that to keep the batter warm. When u grind the batter , pre heat the oven in 90c for 10 minutes and switch off the oven.Now keep the batter covered inside the oven and turn the oven lights on.. It helps .. Add 1/4 tsp of baking soda or baking powder to the ground batter and then ferment. It will help the batter to raise well in harsh cold conditions.. Allow at least 8 - 12 hours to ferment. If you don't see tiny bubbles the next morning, the dough will not rise and you have to start all over again. If nothing works for u , try adding a fermentation starter like a spoonful of commercially prepared idli batter as they have live culture, it helps for fermentation.. Use chlorine-free water when grinding. Tap water will work, but will inhibit the fermentation a little. Most modern water filters remove both chlorine and chloramine.Use spring water, boiled or filtered tap water to avoid Chlorine/Fluorine. Use non-iodized salt such as ‘Kosher salt’ .I always use crystal salt while grinding the rice.By this way,salt mixes with the rice uniformly.