A good jam is best achieved using a slow process of cooking the fruit on medium-low. The sugar and lemon juice helps break down the fruit and cook in its own juices. This is an important process of jam making and natural pectin development. If you cook it on high all juices will evaporate leaving the fruit still firm and not jelly-like pectin. All is not lost and you can still continue to make the jam. Just add a 1/4 cup of water and continue to cook the jam until you reach perfect jam consistency