Annie - Tibbers stun

I don't know if Annie was changed or what, but I have experienced a problem twice now when playing her. In a match I just played, and on last night I would go in with stun up and Tibbers multiple people. Twice I have noticed that it didn't stun everyone. For example, in my mos recent match Maokai and Nunu were right next to me, and I dropped a Tibbers stun to top Nunu's ult and to try and get away from Maokai. However, it only stunned Maokai, and Nunu continues to channel his ultimate. I know for sure that my stun was up because i almost never use my spells on anything but a champion when my stun is ready, and the team fight had just begun. I don't have any video to prove what occurred, but I thought I should mention it in case it's a problem with LoL and not operator error.