When canning, do not expect jams or jellies to thicken as they cool. They should be the desired consistency and/or specified temperature before removing from heat and preserving. In general, 220 degrees fahrenheit is the temperature a jam or jelly needs to reach to gel. You can also use a frozen plate or spoon to check the consistency. Learn more about troubleshooting jams and jellies.  Personally, I like to allow my jams and jellies to really simmer down and thicken up. When you read: “bring the mixture to a full rolling boil that does not cease when stirred”, that means it needs to still be at a rolling boil as you stir it.  Apple butter is traditionally cooked for a very long period of time. I simmered mine for a few hours during step 3 of this recipe.