Why is my milk not setting/coagulating?

If your milk has not set, it could be for a number of reasons. Here are a few: 

• The type of milk you are using - make sure it's as fresh as possible and is the correct type for the recipe (eg unhomogenised/homogenised/UHT). See this article for information on the different types of milk.
• Too little rennet was used.
• Your rennet has expired or has deteriorated due to incorrect storage conditions.
• Rennet activity was destroyed by diluting the rennet with warm water or with chlorinated water - we recommend boiling the water and letting it cool - even if you use tank water, this would be advisable to be sure nothing else in the water hinders the rennet.
• Your thermometer is faulty, milk temperature is too low or too high.
• Your thermometer was not correctly submerged in milk (milk needs to be covering the indentation points) or the thermometer was touching the bottom of the pot, and therefore reading incorrect temperature. 

See here for tips specifically on yoghurt not setting.

See here for tips specifically on cultured cheese not setting.