I tried making sour cream again twice last week. Both times, it failed. I wasn't sure the first time if I'd messed up the measurements and that was the problem, since I had just guessed at the amounts from memory. So the second time, I did it by the book...and it still failed. So I had to wonder if it was the milk I was using. The last few times I've gone to the store, they were out of the local milk so I bought a different brand. I checked the milk we were using and it said "pasteurized" but not "ultra-pasteurized", so it should have been fine. Still, it wasn't working. So when I went shopping over the weekend I bought the last half-gallon of local milk from the store and tried sour cream for the third time. This time, it set beautifully.
Lesson learned. From now on, I will not be making home dairy products except with the local milk. And now we can finally make the moose stroganoff I've been pushing off for over a week since we didn't have any sour cream and I'm now refusing to buy something I can make so easily. :) Yum!