Annie Ult (Tibbers) Won't Stun!!!

It seems that Tibbers's stun is not working properly. I know that it will not stun if I miss, but even when I don't and my stun is up, my ult doesn't stun my opponents. I can't play her mid anymore because sometimes I expect to land a stun on a group of 4 or 5, but no one gets stunned. I wish I could've reported this earlier, but I didn't hoping this would get resolved. This somehow doesn't happen every time, but happens about 40% of the time for me. Is anyone else seeing this problem?