Question

why does idli batter take longer to ferment in winter​

Answer 1

Answer:

because in winter it freezes instead of ferment

Answer 2

At times, not adding enough salt to the batter after its ground can also be a reason for lack of fermentation. Try to add at least 1/2 tsp (my mother mother suggests 3/4 tsp- 1 tsp) for every 1 cup of dry ingredients (rice +urad dal). You might probably need to adjust later according to your taste.

Explanation:

to make fermentation faster during winter follow the above