Some fruits have calcium in them naturally. This natural calcium will activate the pectin. Wait until the next day to see if your mixture jells when thoroughly cool. If it has gelled, you don’t need to do anything.
If it hasn’t gelled, open the jars and put the mixture into a sauce pan. (If you made the jam or jelly recently and you carefully remove the lids without damaging them, you can re-use the same lids.) Add the missing calcium water and stir well. Bring the mixture to a good boil. Boil and stir for 2 minutes. Jar and process according to the recipe sheet that comes with Pomona’s Pectin.