Annie stun sometimes broken?? O.o

Lately I've been playing Annie again and I'm doing really good, but sometimes I've noticed that Annie's "stun" doesn't really "stun" the enemy champs. I played a lot of games when that happened, sometimes is life or death, or kill for myself or a setup for a gank. I was really mad when i was play against a and he was under turret and i ulted on him WITH stun up, but he just kept on walking like nothing happened. i was wondering if this happened to anyone else.