Eve W Proc

I was playing a game as Eve against an Ivern and when my W charm was ready, I hit hit with my Q. I know for a fact I hit him with my Q because the three orbs of extra damage was circling him. Despite this, he wasn’t charmed. Now he did have a shield on him but I didn’t think you had to lower their health bar to charm them. Was it just a fluke or is this how Eve’s W works?