After watching the professionals making joghurt in the Flumserberge, I, like them, add dried milk powder (Magermilchpulver - from the Migros) to the litre of milk.
i.e. 1 Liter of Milk, 1 Bio natural joghurt, six teaspoonsful of dried skimmed milk powder.
When I first made our joghurt, I used Pasturised Milk. About five years later, while living in England, my joghurt seemed a bit slimy and someone suggested UHT milk, which I have used for it ever since.
I add no sugar and no fruit and I often don't warm the milk. If I mix the ingredients straight after breakfast with milk at room temperature it is ready before I go to bed in the evening.
Maybe the addition of powdered milk would help with the Bio milk too. A Reform House might even have Bio Milk Powder.
Good luck