I use a very simple method.
Bring milk to a boil Boil for 5 minutes (it will kill off bad things present in the pan or the milk).
Cool the milk down to 115.
Pour it thru a strainer into the starter bowl (which you made impeccably clean before adding the starter). Cover with plastic.
Keep it warm at that temp or thereabouts using your preferred method. I have a heating pad that happens to do the trick, with the towels I've wrapped it all in.

I make yogurt from a gallon of milk every 3 days and have never had a problem.

1. Heat milk to 180, stirring to make sure there's no scorching. (I'm impatient, so I prefer to stir over high heat for 11-12 minutes, always scraping the bottom, rather than waiting 30 minutes and stirring occasionally.)

2. Allow to cool to 115. (I use the refrigerator for about 30 minutes.).

3. Stir in 1/2 cup yogurt.

4. Put a plate over the pot. Wrap in a thick bath towel and place in your microwave, which is airtight.

5. Start checking after 4 hours to see if the whey has separated. (I prefer to catch it just as it's separating, so that it's not too sour.)

6. Remove the plate and refrigerate for about 8 hours. The curd will have time to firm.

7. Let drain through a sieve lined with plain paper towels or cheesecloth in the frig until you have the thickness you desire. (I like very thick Greek style yogurt, so I drain off about 10 cups of whey, from a gallon of milk.)

8. Pour yogurt into a container and enjoy. Use the whey in soups, salad dressings, etc. I use the whey to create "slaws" with grated carrots and onions, or grated cumbers and onions by adding some vinegar, mustard, herbs, etc.

Buying commercial yogurts is a tremendous waste of money. Yogurt making takes about 20 minutes of hands on effort. And nothing beats the taste of freshly made yogurt.

This used to happen to me all the time, until I switched from using 1% or 2% to whole milk. Problem solved.

When we were getting raw milk, the dairy farmer said to remove as much of the cream as possible before making yogurt with it. I finally just gave up. The homogenized milk just made better yogurt.

also, try adding powdered milk to your milk mixture along with the starter-- just a quarter cup per gallon of milk. at first i thought the idea of adding powdered milk was gross, but it really does improve the texture.

I have a yogurt incubator and it always seemed to turn out great...I buy freeze dried culture at the vitamin store. I haven't done it in a while though....once my kitchen is back in working order from renovations I will get into making it again. I will try using whole milk...I always used 2 percent but I am sure whole milk is way better.

Here's a video we put together on Making Greek Yogurt . It's super simple...https://vimeo.com/34541592

I had the same problem...tried different type of starters but not helpful. Many Indian families set yogurt at home. I borrowed an ounce from an acquintance an year ago. Works with all types of milk. Fat free, 1%, 2% , whole, half and half too.
I boil milk in a glass pyrex bowl and microwave it till it nearly boils over(a quart and a half about 12min). Let it cool till it is lukewarm/warm. Add yogurt culture about the size of a M&M. Cover it. And store it in a warm place.
Couple of things though. If milk is hot when culture is added, it will kill off the bacteria in the culture and yogurt will not set. If milk is cold, the yogurt will become elastic. Just heat the milk for 2min in microwave again if cold.
I usually set the yogurt last thing in the night. If the weather is cold, I heat a corningware bowl for 2min in the microwave and put the pyrex bowl in it and cover both. Yogurt usually sets by 5am. The longer i leave it out, the tangier it is. The fat content in the milk generally determines how thick the resulting yogurt is.
All said and done. Finding the right culture is important. I poured down the drain lots of stringy yogurt before i asked an Indian lady i met, some yogurt culture.
Ten days too late. Hope i could be of help.