Well you could - back to full boil, more pectin, re-jar and re-process - if you wish. But honestly in the opinion of many it isn't worth the time and effort and energy expense. It either sets so hard you have to slice it or it only gets thinner or it develops a scorched flavor. Best recommendation - just plan to use it as plum syrup for pancakes and/or as a dessert topping.

Next time use more under-ripe plums for a better set as they have much more natural pectin.

Dave