Wasp passive stun not working....

To my understanding Wasp's stun that she activates during a combo is passive and cant not altered or countered, and is supposed to be 100% of the time.... the stun can even trigger on champs like Mordo, stun immune nodes, etc. However, during my fight with the AW boss today I attempted to use that and it worked less than 50% of the time. Can you please look into this and make sure its working as intended? Thanks.
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I also think it is affected by ability accuracy as well, since passive abilities are in general affected by it.