Shyvana's Ult not knocking enemy champs back

I just played a match as Shyvana and every time I'd ult in to gank it wouldn't knock the enemies back to where I'm going, I know it's not that I missed since I ulted someone when I was right next to them.

edit;; was only happening some of the times (still playing the match actually and just ult'd riven and it worked but the other times it didnt)