If you wake up to unfermented batter, it is usually recoverable rather than ruined. Moving it to a warmer spot and giving it another four to six hours often rescues it entirely, particularly if temperature was the original problem. If it still refuses to move, a small pinch of baking soda or a spoonful of active curd stirred in just before steaming can provide enough lift to salvage that batch, though this is a workaround rather than a real fix. You can also repurpose unfermented batter by thinning it slightly; it makes perfectly good uttapam or paniyaram, so nothing needs to go to waste.
How to tell when fermentation has actually worked
Knowing what success looks like helps you troubleshoot faster. Properly fermented batter roughly doubles in volume, develops a visibly bubbly, porous surface, and gives off a mildly sour, pleasant aroma. Stirring it should feel light and airy rather than heavy. A useful test is dropping a small spoonful into a bowl of water, well-fermented batter floats, while under-fermented batter sinks. Over-fermented batter, by contrast, smells sharply sour and may have collapsed after rising, which usually means it sat too long in excessive warmth.
Getting consistent results over time
The encouraging part is that idli fermentation becomes reliable once you identify which variable your particular kitchen is struggling with. Most households discover that a single adjustment, usually temperature or dal quality was the persistent issue all along. Keeping notes on soaking times, grinding method, and where you placed the batter helps you narrow it down quickly. Once those conditions are dialled in for your climate and equipment, the batter tends to behave predictably, and mornings of flat, disappointing batter become a thing of the past.