Taunt failing

Recently--for about the past three patches--I've noticed my taunt occassionally failing to have an effect.

I play a lot of Shen, and while the taunt "aura" always occures on targets I hit, sometimes they just continue running in the direction they were going, only moving toward me in the last half second, or even simply continue attacking my teammates instead of me. When the taunt only lasts 1.5 seconds, having more than half of that effect not occuring is kind of bad.

Why don't my enemies care about taunt?