For Yogurt to set two things are Important: Warmth - Once you add the culture to the milk - you need to keep it a warm place - no moving no shaking. I find a warm switched-off oven works best for this. In winter, it can take longer. Also, if my oven is cold in winter I turn the oven on at 30 C / 70 F for five minutes. Then, I switch it off, and I place the yogurt jar in the oven overnight. Perfect temperature for making yogurt. Do not disturb - Do not touch the jar while the bacteria culture is doing its job. Leave it alone. No peaking.