Fix D: Measured whole berries or pieces of fruit instead of mashed fruit
First you will have to figure out how many cups of mashed fruit are in your jam. When you measure whole berries or pieces of fruit, they will generally mash down to half the amount you started with. If you measured 4 cups of whole berries or 4 cups of pieces of fruit, you probably have 2 cups of mashed fruit in your jam. You need to add 2 more cups of mashed fruit to your jam if the recipe called for 4 cups of mashed fruit.
Fix your jam: Empty jars into sauce pan. Add the needed cups of mashed fruit. Stir to combine. Bring the mixture to a full boil. Stir vigorously for 1 minute to dissolve the pectin that is already in the mixture.(If you want to add more sweetener, you can do so now that the pectin is dissolved. Add sweetener and stir well.) When mixture returns to a full boil, remove from the heat. Jar and process according to the Recipe sheet that comes with Pomona’s Pectin.
This link will take you to the complete page My Jam or Jelly Didn’t Jell — Can I Fix It? if you haven’t seen it yet and want to read more about possible problems and their solutions.