Homemade Greek Yogurt

Hello, I'm my own yogurt. I add Voskos Greek yogurt, simple, original, milk .. On two occasions he did not work .. I heated the milk to the proper temperature, added to the culture temperature and incubated approprate the right length of time .. However, I am not able to make the plain yogurt with Voskos ... An experience similar negative results with this?

Are you sure you did not add the yogurt before the milk cools to 115 degrees F? I did several times I have used yogurt and raw milk. Here's how I make my yogurt:

Boil 1 liter of milk (raw or pasteurized) to 180 degrees. Remove from heat and let cool to 115 degrees. Add 2 tablespoons packed store bought yogurt. I also add 1 c. tea (plain envelope Knox gelatin as I like my yogurt to the consistency of such purchase), mix, then placed in containers (I have a yogurt maker), seal and heat for 8-12 hours , by how tart you like your own.
I know you can make yogurt in a pot as my friend does it all the time, and also in the oven, but I invested in a yogurt, I like the convenience of having the constant heat 115 degrees.
I usually do mine at night, it is ready the next morning.
If this does not work, then try to use the entire container 6-8 oz. If that fails, then I would say that your unit is not functioning properly. I've never had yogurt fail. And I usually do with the first batch purchased in store, lot or two with mine, then store bought again.

Make sure your starter yogurt has live cultures. If you have boiled the milk before and kept the temperature for the exact amount of time, it should work.