I wish there were a simple, straightforward answer, but there isn't.
I can give you some hints that will help to ensure you make a successful batch of yoghurt every time.
This is my MOO yoghurt recipe.
To make 1 litre of yoghurt you will need:
A clean 1 litre jar with a screw top lid
A wide mouthed thermos that will hold the jar
2 cups milk powder (either full cream or skim, it’s up to you)
2 tablespoons natural yoghurt (this is the starter)
Cool water (tap water is fine as long as it’s not really cold).
Half fill the jar with cool water, add the milk powder and the 2 tablespoons natural yoghurt. Put
the lid on and shake well to combine. Make sure there are no lumps of milk powder left. Fill the
jar to the top with more water, seal and shake well. Shake it really well, for at least two minutes. You need the starter and the milk powder to completely dissolve.
Place the jar in the thermos. Pour boiling water around the jar until it reaches approximately 2/3
of the way up the sides. Put the lid on the thermos, sealing it tight.
Set aside for 8—12 hours, depending on how thick you like your yoghurt. Once it has reached the
consistency you like, place the jar of yoghurt in the fridge to cool.
First off, check your thermos. I have two: Easiyo and Aldi. These thermoses don't seal tightly, but the lid does sit right down on them. If the lid on your thermos doesn't sit right down, it could be leaking the heat needed to set the yoghurt.
Did you use skim or low fat milk powder? Commercial yoghurts have thickener added, that's what makes even the low fat versions "thick and creamy". If you like a thick and creamy yoghurt then full cream milk powder is the answer.
Is your starter old? If you've been saving some of each batch as a starter for the next batch you may find that over time it will weaken and finally just not work. After every five or six batches I suggest you use a fresh bought yoghurt as your starter. If you don't want to eat the leftovers, you can freeze it in ice-cube trays to start other batches.
If your yoghurt isn't as thick as you like after 12 hours don't stress. You can safely leave it to set in the thermos for up to 24 hours. Your yoghurt won't stop thickening until it is chilled.
The weather can play a big part in a good set or a runny fail. Too hot, too cold, too humid can all determine how long and how thick your yoghurt sets. In winter try wrapping the thermos in a towel for extra warmth and then topping up the boiling water every six hours or so.
If your yoghurt doesn't set as thick as you like, don't throw it out and waste it. Use it to replace the liquid in cakes, pancakes, muffins, white sauce or custard or add it to milkshakes or smoothies. .
Don't give up. At less than half the price per kilo of even the cheapest bought yoghurt, it is well worth persevering and fine-tuning your yoghurt making skills.