Some fruits, such as Granny Smith apples and gooseberries, contain enough pectin to thicken into preserves and marmalades with just a little sugar and heat, while others, such as oranges, need extra pectin. Orange marmalade relies on an exact proportion of pectin to sugar and acid to activate, otherwise it won't gel and advance past the heavy syrup stage. You can fix runny orange marmalade if you add more pectin, but how you add it depends on which type you used, powdered or liquid. Unlike jellies made with high-pectin fruits, you can't fix orange marmalade with heat alone.
Using Liquid Pectin
Portion 1/2 tablespoon of pectin, 1/2 tablespoon of store-bought lemon juice and 6 tablespoons of white granulated sugar for every 1 cup of marmalade you need to remake and set it aside.
Bring the orange marmalade to a boil in a heavy-bottomed saucepan over high heat while stirring continuously with a heat-proof spoon.
Take the marmalade off the stove when it starts boiling and quickly stir the sugar, lemon juice and pectin into it. Return the saucepan to the stove and let the marmalade reach a rolling boil, stirring continuously.
Boil the marmalade for one minute, still stirring continuously. Skim the foam from the marmalade with a heat-proof spoon and discard.
Allow the marmalade to cool to room and store it in a food-storage container in the refrigerator. If preserving, fill sterilized canning jars 1/4 inch from the top with marmalade and process them in your canner.
Using Powdered Pectin
Add 2 teaspoons of powdered pectin, 1 tablespoon of store-bought lemon juice, 1/4 cup of water and 4 tablespoons of white granulated sugar and to a heavy-bottomed saucepan for every 1 cup of orange marmalade you need to fix and bring it to a boil, stirring continuously with a heat-proof spoon.
Spoon the runny marmalade into the saucepan with the other ingredients after they start to boil and stir to combine. Let the marmalade reach a boil, stirring continuously.
Boil the marmalade for 30 seconds, still stirring continuously.
Skim the foam from the marmalade and allow it to cool. Store marmalade in a food-storage container in the refrigerator or pour it into sterilized canning jars while hot and process in your canner.