Is your yogurt raw?
Raw yogurt is kind of an anomaly: To establish a high amount of yogurt culture you need to kill all or most of what is in the milk originally or it will not culture well.
The second reason: It would not be yogurt then, but a kefir instead.
Third reason: raw yogurt is not allowed as a legal dairy product. Only two dairy products are allowed In PA in the raw form and those are milk, and cheese that has been aged for 60 days.