I think making yogurt is a very subjective art, and kind of fickle, honestly.
There are times my yogurt just doesn't 'yog' for some unknown reason.
maybe the milk is too hot and kills the culture
maybe there is some bacteria on the inside of the jars
maybe the chest doesn't hold the heat well enough
maybe it sits too long
maybe it sits too short a time
maybe some competing microorganisms got into the mix
maybe it's the milk, and a different brand/animal/? would change the outcome
Hard to say. I just keep at it because healthy probiotics with some kind of healthy dairy seems to be essential for my gut health. Right now, I've found a good source of clean, raw, local goat milk, and the Polyflora B is working well. Otherwise, I'd just keep making yogurt, using the best choice of milk that I could find; and use a ready-made yogurt with live culture in it. Or I'd buy packaged culture if I had to. I just keep on ticking, like that Energizer bunny!