A friend gave me this recipe for yogurt -- a process she used regularly in her house. She argued that this recipe was foolproof. When I have followed her instructions, it has worked every time. I have tried other homemade yogurt recipes that have not set up / solidified. The differences have not been in ingredients, as much as technique for keeping the yogurt consistently warm over a long period of time. I find my friend's method to be foolproof.
You could always add yogurt starter, if you wanted to guarantee results.
1 1/2 gallons whole milk
1 c yogurt, with active culture
Boil milk. Use low flame so you don't burn the pan. Boil for 2-3 hours.
Cool to baby bottle temperature.
Add 1 c yogurt. Mix.
Put in glass jars with lids or plastic jars with lids. (In this picture, I've just nestled the jars in a cooler filled with warm water, to keep the temperature consistent as I filled the remaining jars.)
Put whole thing in gas oven which has been preheated to 300 and then shut off. Leave for 6 hours.
After 6 hours, take lids off and put jars in fridge.
When they are cool, put lids back on.