I only use the powdered stuff and the low or no sugar type. I just can't bring myself to put more sugar than fruit in my jams and jellies. I do however use the liquid Certo for my pepper jelly because the finished product actually looks better.
I'm wondering...Did you double up your recipe because this often causes the pectin not to set or has your pectin expired? Does pectin expire, I don't know if it does or not. Well anyway, just a couple of thoughts. I had muscadine jelly not set up because of one of these reasons and I use low-sugar pectin but if made great pancake syrup.
I made 4 pints of blackberry jelly today but it looks like it set up pretty good. If I have time tomorrow I'm going to make some elderberry syrup for flu season. Some kids at our high school have already tested positive for it.