Riven W stun not working

I just played Riven into a Renekton, and over 10 times (that I counted) I would stun him with my W, and he wouldn't et stunned. I found that it seemed to be when ever he would be in the channel of a skill. If he was in the middle of spinning in his Q, slicing in his W, dashing in his E, or getting big in his R, and I used my W to stun him, it wouldn't do it. I don't mean it just didn't cancel the animation, and it stunned him after the animation of his skill finished. I mean it wouldn't stun him at all. I died 5 times because of it.

And on the note of renekton, he had a Dorans Blade and Long Sword and that was it. But (with no fury) he would do 350 damage with a Q, and a 350 with a W. It was un beatable as he 2 shot me, and couldn't be stunned.