For an immediate fix, gently stir some additional sweetener into the jar.
Another option is to empty the jar into a small saucepan. Add the sweetener you want to the mixture in the pan and bring to a boil, stirring well for 1 minute. Put the hot mixture into a clean, hot jar and put on a washed lid. Eat when thoroughly cool and the mixture has re-jelled.
If you want to fix a whole batch at once, pour the contents of all the jars into a pan – remove lids carefully if they haven’t been on the jars for very long and you want to re-use them. Add more sweetener. 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.