Text reads: “at some point in your life you will be boiling fruit, water, sugar, and lemon juice in a pot to make a syrup or jam. The instructions will tell you to simmer for a certain period of time. Your timer will go off and you will look at the pot and go “hm this doesn’t look thick enough, maybe I’ll let it go for another 10 minutes”. This is the devil speaking. It’s only so liquid right now because it’s at boiling point. It will thicken when it cools down. Learn from the follies of my youth and do not let this happen to you.”
I usually try to salvage the jam immediately, so I don’t know. You could try re-making one jar and see if it sets?