I was playing on my Swashbuckler yesterday and noticed that it seems as if her power card is fizzling. Then I noticed it again this morning.
For instance this is how the battles went: She goes hidden with Walk in Darkness. The next turn she will use Assassin's Gloom (5 turn spell) which shows the vine animation and the notification shows up on the enemies that are poisoned. Then the next turn or soon after (depending on when Assassin's Mist shows up), she uses it on her enemies. Only now there is no animation and no notification of additional poisoning on the mobs.
It almost sounds like you are trying use Assassins Mist while Assassins Gloom is still active. Poisoning and Bleeding don't stack. This means that you can't use another Poisoning (or Bleeding) power while the first one is still active. However,. you can use Bleeding while Poisoning is active or Poisoning while Bleeding is active,