Crowd Control not sticking

I just got out of an Udyr match where neither my bear stance stuns nor Velkoz' knockups were having serious effect. My presumption throughout the game was that the enemy Garen was possibly running 9/21 masteries or something with bonus tenacity (my bear-stance cc wasn't even making him stutter mid-fight). Post-game analysis reveals that he had literally 0 tenacity, so I'm completely confused. There was one fight in particular where I hit him with bear stance and he just kept running through the team (I will add that we had already popped his Banshee's). In two distinct instances, he ulted approximately 1/2 a second after I stunned him. After I stunned him.

In another slightly less troubling incident, I bear-stunned Rengar (after he jumped my Velkoz), and he paused before continuing his fight. The stun marker was on, but it was less than 0.25 seconds before he continued his fight (literally just canceled his basic attack). Something felt extremely bugged about the entire game, because almost all I could do was CC Thresh, Syndra, and J4. Everyone was noticing the "bug," but the enemy team said it "has always been like that," and thought we were weird for being surprised that cc wasn't sticking at all.

It felt very inconsistent, too -- like, sometimes it would work just fine... and sometimes it would do literally nothing at all (except, in my instance, place the stun marker on them). I'll try to collect any more info I can get if it happens again, but it was really unsettling in-game.