To Cloverdale: I know your post is a year old, but I just wanted to say that, yes, there actually ARE do-overs when it comes to jam that did not set well. You just dump all the jam out of the jars into a jam pot. Bring to a full, rolling boil. Cook for just 30 secs. Remove from heat. Add in more pectin.Add more pectin (I would suggest perhaps the rest of the package?). Stir and skim for 5 minutes. Put back into clean, sterilized jars.
Note that liquid pectin called for in this recipe and the powder or crystal kind are no interchangeable. The crystal pectin generally calls for less fruit and less sugar for 1 packet than will be called for in a recipe using liquid. Also, double check the size of the pectin. It used to come in 6 ounce bottles, which is what this recipe is referring to. It calls for 1/2 of a 6 ounce bottle or 3 ounces. Today, liquid pectin is usually in 3 ounce pouches, so you would use a full pouch.
If you want to make jelly, you cannot just substitute an equal quantity of juice for the fruit. Jelly making is another whole art and uses different recipes. Usually more sugar and more pectin.