Fiora's W - Riposte isn't stunning properly

I've noticed on several different occasions in my recent games as Fiora that, when I parry an immobilizing CC and hit a champion with the riposte, it does not consistently stun the target. At first I was unsure if I had just missed, but the problem has recurred enough times that I'm convinced it's a bug. I know I didn't miss either because it still applies the movement and attack speed slows - it just doesn't apply the stun. Has anyone else observed this issue?