I'm trying to make yogurt, but it won't turn out. Can someone help? I followed this recipe. It follows the pattern outlined here but with different temps:  


Get milk. – I’m using 1 cup of 2% (no powdered milk).
Get a starter. – I have tried Dannon Oikos plain greek yogurt (made with whole milk), and Yoplait Very Vanilla, and some other brand of plain greek yogurt. The “other brand” yogurt was at least a couple weeks in the fridge before I tried using it for starter.  
Heat the milk to 175-180°F ( instead of 190°F).  
Cool the milk at to between 40°C (110-120°F).  
Mix in the starter. (I used ¼ tsp for 1 c milk.)  
Keep the mix warm for 6-10 hours. – For the heat source I used a crock pot on “keep warm” with 2-3” water. The milk/starter was loosely covered in a class measuring cup. I monitored the temp with a thermometer and kept it around 110°F. (It probably varied from say 105-115° as I tried to keep the temp 110°)  
Strain your yogurt. – The one batch that solidified, I successfully strained with a coffee filter. My problem is just getting to this point.  


The first time I made it, I heated my oven to 170°F (as low as it goes) and then turned it off. I wrapped the still warm milk/starter (in a glass bowl) in towels in an oven that had been heated to 170°F and then turned off. I used the “Very Vanilla” starter and in the morning it was still milk, so I added some plain greek yogurt and transferred it to a crock pot on keep warm, with the crock pot lid on and a towel over the lid. After several more hours it had solidified but was beige colored. At that point, a temp check revealed it was 170°F. Oops. After straining it, the texture resembled ricotta cheese. It smelled nutty and delicious, but didn’t taste nutty or delicious. I think it was getting all that smell from the whey. The taste was not unpleasant, but not pleasant either, and definitely not tart like yogurt. 

The second/third times I made it, I just used plain greek yogurt starter. Incubated in a crock pot with a water batch for 8-10 hours. The result was milk. When I poured out the milk I could see some solids (what looked like the starter) at the bottom.

So now I’m wondering what went wrong. Several questions come to mind:


Is my starter too old? If so, how do I get fresh yogurt?
Am I using enough starter?
Am I getting the milk hot enough in step 3?
Am I keeping the milk too hot/cold during incubation?
Does the humidity from the water bath help/hurt my chances of producing yogurt?  


Can anyone offer some guidance? I have a hunch that the starter is too old since I don’t really know how old yogurt is when I buy it from the store. But I also don’t know how to do any better than buying it from the store and making yogurt right away. 

There are so many variables I’m not sure of that I just thought it would be good to get some advice before proceeding.
Don't get frustrated. There really aren't that many variables to keep track of.

1- I don't know the age of store bought yogurt but I have never had just-purchased yogurt not work as a starter. I have had month old yogurt from my fridge not turn out. If you don't plan on making yogurt often enough to keep you starter viable then consider freezing some in ice cube trays.

Whatever starter you use- make sure it lists "live, active cultures". Personally I wouldn't use a flavored yogurt as a starter.

2- I don't think that is enough starter. I will use 1/4 cup of starter for a quart of milk. If you are using 1 cup of milk that would be one Tbs of starter (12X the amount you used).

3- It depends. The goal is to denature the albumin proteins in the milk. 180F is hot enough if the milk is held at that temperature for at least half an hour. It needs to be held at 190F for closer to 10 minutes. 

4- You need to get better control of your temperature. 130F will kill your starter but erratic temperature fluctuations will also produce very poor yogurt. Without a temperature controller or manual intervention your slow cooker will get much too hot- even with a water bath.

More successful approaches are to put it in a draft free place in an insulated container, or in your switched off oven.

5- The humidity is not going to play a role unless it is condensing into your container enough to dilute things.

Good luck. I hope you get it working. Fermenting milk is fun.