Using it from stealth with ambush on sin rogue. It applies to an enemy or 2 then after that it doesnt work once or twice then it starts applying again. Its totally random when it works and doesnt work.
I am talented for it and am using it from stealth via ambush.
EDIT - I just figured it out. Its not applying when the enemy isnt targeted aka the ACTIVE target and Im relying on the action targeting to attack. So if I still have a dead enemy targeted and I sneak up and attack another enemy without changing to that one as my main target it doesnt apply.