There are multiple factors that might be responsible for idli batter not fermenting. Few are as below: 1. Use of old stocked Urad dal.  2. Runny or thick batter - If the batter is not at the right consistency, too thick or too thin, the batter will not ferment or rise.  3. Use of iodized salt can hinder fermentation so always use non-iodized salt.  4. Not enough salt can be another reason. Typically 1/2 to 1 teaspoon of salt per 1 cup of dry ingredients is recommended but adjust to your taste.  4. Batter got warm while grinding. Add ice-cold water while grinding to prevent this from happening.  5. Temperature wasn’t warm enough to ferment. Place the batter in a warm place for fermentation.