Slow Poison as a Hero Ability? Having trouble.

I've edited Slow Poison to a hero ability, adding a couple of levels to make it like a normal hero ability, and gave it to my hero. Unfortunately, no matter what level I'm at, the hero's attacks never poison. The buff icon never appears on the enemy, and they never take any damage over time. I'm sure I'm missing something simple, but just can't think of what it is...

I know that certain types of attack (missile, bounce for instance) do not work with slow poison nor envenomed weapons - try changing this type to normal or missle and see if your hero suddenly becomes virulent again.