Annie/Tibber stun doesn't work all the time

TITLE. It seems every time I get annie in ARAM, I get my stacks ready so I can toss tibbers and stun. At least one time each game, I toss Tibbers and he lands but won't stun. My stun charge got consumed though. WTF?! Has been happening for the last 2 weeks I would say. 95% of the time stun works but man, when the flash/snowball tibbers toss doesn't stun, it = dead Annie fast!