Did you use anyother pectin? Here n the U.S. we don't have a product such as "jam sugar" which must be strictly a U.K. product. I make preserves and jellies using regular, granular sugar and the recommended pectin.
One can, of course, not use pectin at all and simply cook the fruit/sugar mixture to the "sheeting" stage using a wooden spoon.
Additionally, did you sterilize your jars? At least 10 minutes completely submerged in boiling water or ten minutes in a 350 degree (F) oven...
Better luck next time!