Poisoned weapons not working??

I just bought a Hashishin sword with poison, but I don't get it working.
Can you only poison certain enemies?

I've read on WoG that you only poison enemies with the stab-attack (Hold right-click and long left-click) but that doesn't seem to work either.
Do you first need a skill for it?

I know that fire DoTs work properly, but they only have a low % chance of being applied. I've never gotten poison to work, but I suspect it only has a low chance of being applied as well. Also, I've never seen any NPC get poisoned at all - if I bring Angar with me and let him kill a dozen people with poisoned knives/daggers he will never get poisoned, but I might get poisoned at the first blow.

Right its percentage I am guessing.
I have used Poison Arrows and not every Health bar turns green but it does happen.
Could be natural resistance for some also, if PB had time to implement it.
THe only one that stands out so far is a Fire Golem I tried to used Fireball on and it didn't work so I used Ice Lance.
I heard someone used Fireball on Fire breathing Drgaons and did work but that was pre Patch 2 whick could have implemented Resistances.