There might be two reasons. Either you didn’t add enough sugar or you didn’t cook it longer. You have to take the drop test mentioned in the recipe notes. It will give you an indication that the jam is cooked and is ready to be stored. If using a cooking thermometer, make sure it reaches 220 degrees F. It may take 24 hours for a jam to be fully set. The jam will be set more if you place it in the refrigerator.