"mobile" wrote in
. net:

pectin recipes, with sure jell powdered pectin, check date
on package, new stuff,

over ripe fruit can make soft or runny jelly/jam. if you
have, say, 10 pounds of peaches & you make a peach jam the day
you get them, and then use more of the same peaches for a
ginger peach jam 3 or 4 days later, the peach jam may be fine
& the ginger peach may not set, because the peaches have
continued to 'ripen'.
i think humidity has something to do with it too.
lee