I was looking in Bittman's How to Cook Everything for something else, and came across this information and thought of this thread. I have never made jelly, so maybe this is commonly-known information to those who do, but thought I would post it in case it helps:
"If you want to make absolutely certain that your jams gel firmly, add 1 teaspoon of liquid pectin (such as Certo) per cup of fruit."
"You can determine in advance whether your fruit will gel: When it has cooked down to a mush, place 1 teaspoonful on a cold plate (freeze the plate for 10 minutes first, or dip it in ice water; dry it before proceeding). Place the plate in the refrigerator, and look at it in 2 minutes; if the jam has gelled, you're all set. If it hasn't, cook a little longer, adding a little liquid pectin if you like. Or use an instant-read thermometer: At 224F, all jam will gel."
It doesn't matter what you think, just that you do.