I would try a different culture. Even though good store-bought yogurt brands are great themselves, I've never had good luck making my own yogurt with them. It's always too runny, too sour, etc. I buy the 'sweet' yogurt culture from here: http://www.cheesemaking.com/cheeseculturesandmoldpowders.html
Don't listen to them when it says to use the whole packet for 1 or 2 qts! You only need a bit.
I use a similar yogurt maker, but mine runs a little too hot, especially right on the bottom where the tub is in contact with the warmer. That's what gives you the 'cheesy' cooked texture at the bottom.
Here's what I do: warm a quart of milk to 90* (I have a cow, so it comes into the kitchen at that temp), then pour it into a quart mason jar. Add a sprinkle of yogurt culture (no more than 1/8 tsp), screw on the lid, and give it a good shake.
Place an extra lid in the bottom of your yogurt warmer. This will keep the jar from directly touching the hot spot, and results in a more even temperature.
Put your jar in, and cover it with the dome lid. I usually let mine go for 12 hours, but with another cow's milk it required 24 hours to set.
I hope this makes sense!