15% is a low chance in terms of mobs and players.. Say for instance the chance of Spirit Blackhole purging a target. My archer can hit someone for 3 minutes straight without purging once, but then again, I can go hit a single mob once, and it'll purge. It's just about percentages and chance, and how often the skill itself procs. Demon vortex isn't broken last I checked, so it's probably just the fact that you've got really bad luck at the skill proc'ing...