Pantheon stun(Aegis of Zeonia (W)) not working

I played a couple games as pantheon today: In the first one I was against a Quinn, if I cast my stun (w) and while in mid air Quinn casts her vault, she does not get stunned in the spot or after landing from her vault. In the second game I attempted to stun a Yasuo and for no reason he wasn't stunned. Also in the second game I attempted to stun a Fiora, I landed at her location but instead of her being stunned she then cast riposte. Now I would understand that to be working as intended, except the riposte came out late and I was standing the the riposte area and I was not stunned. Therefor, either my stun once again didn't work as intended, like the above examples, or fiora riposte didn't work as intended. This obviously isn't happening every time but it happened enough times for me to think there might be a bug with Pantheon W.