You're really making me think on this one Amber!
If I understand what Harold McGee said correctly - it takes a balance of heat, time, acid, sugar and pectin to get jelly to gel. Trying to figure this out ... if you followed all of the directions exactly except for cutting everything in half except for the pectin ... then it might
be that there wasn't enough acid and sugar for the pectin to properly react with. I honestly can't say for sure - but that's my best guess.
Concord grapes seem to be the only ones that can be jellied without either liquid or powdered pectins (don't know - every grape jelly recipe I've run across specifies Concord grapes), and some other fruits and berries also require added acid (lemon juice).
One thing I noticed going over various recipes is that if liquid/powdered pectin is used the recipe always calls for more sugar (as much as double) than if made without it. The exception would be that there is a powdered pectin that is designed for low/artificial sugar jams ... and it contains Xanthan and another kind of gum to do the thickening.
Last observation ... several sources state that even when cooked to the proper gelling stage - jelly may sometimes take a couple of days to fully set up. They didn't mention if these were with or without liquid/powdered pectins.
I hope I've helped more than confused.