Tonight we were having meatball subs we open store bought spaghetti sauce and put it on our subs. We just spooned it out of the jar, no heating. We looked away for a moment, then once we started eating I looked at the sauce and it had bubbled and run over the sides of the jar. The top looked like lava with bubbles busting air. Anyone ever hear of this? What happened? It was sauce with Italian sausage. 
you need a Candy thermometer. Sugar needs to be heated to the soft or hard crack temps in order to set as desired. If the sugar does reach the needed temp, then it won't do what you wanted. Hard crack makes hard candies, soft crack makes softer but firm candies. 