Ideally idli batter is perfect to use when fermented for 12-13 hours. Try leaving it to ferment for another couple of hours. If it still does not ferment, simply stop the process. And use this batter with some ENO or baking soda to make Uttapam, Dhokla or Handvo. Do not ferment more than 15 hours else the batter will smell foul and develop a pink crust that is not safe to consume.