Every now and then, perfectly followed directions end up being a runny failure instead of set jam. It may be the humidity, but there is just no rhyme or reason. This may never happen to you, but if it does, here's the fix:
You can redo up to 8 cups of runny jam at a time. The portions are PER CUP of runny jam. FYI - a 1.75 oz box of pectin measures out to 6 teaspoons.
1 tablespoon water, 1 1/2 teaspoon powdered pectin, mix and bring to a boil.
Add 2 tablespoons of sugar and 1 cup of jam and bring to a boil over high heat while stirring constantly. Boil 1/2 minute. Skim any foam, fill your jars and reprocess.
I did 8 cups, so I used the following proportions: 8 tablespoons of water, 12 teaspoons of pectin, 16 tablespoons sugar.
It was a pain in the neck, opening all those perfectly sealed jars and (sigh) dumping jam back into the pot, then washing and reheating all the jars. I had to buy new lids, as the seals are a one-time thing. And I had to buy four more packets of Sure Jell. NOW my jam is perfect. It was so good syrupy, I kept a couple of half pint jars to use as pancake or ice cream syrup. Yummy!