What would cause fudge not to "set"?

I, uh, made fudge. Of course, I had to follow my heart and not the recipe. In a nut shell, I substituted 1 cup of cherry pie filling syrup (WITHOUT the cherries!) for one of the 2 cups of sugar it called for (I used 1 cup sugar instead of 2.) I know I was subbing a sugary liquid (syrup) for a solid (sugar) so I boiled it three minutes longer than than the recipe called for (7 minutes instead of (4) but…it’s not going to set up. Why? Would boiling it longer have gotten rid of the excess liquid and allowed it to set up?

Anyway, if nothing else, I have some kickin’ fudge topping for ice cream! I know I do! Because I also added about ¼ tsp of orange extract, and anyway. It tastes fantastic….but it’s not “fudge.”

I got the original recipe off the internet (sans the cherry pie filling syrup and the orange extract) so…it isn’t my fault it didn’t set up, right?? Why is everything so sticky around here??!!!