What you were expecting to happen, and what actually happened?
The description of the Watcher’s Piercing Gaze says it will drain an enemy’s mana if they have a status effect, but it doesn’t work if they are diseased.
What are the steps to make it happen again?
Happens every time…
Do you have any screenshots or video you want to share with us so we can see the problem? Attach them to your post!