Tristana Alistar interaction [Gameplay]

Unfortunately i wasn't recording the game, and i've no idea if the bug is reproducible. I was playing Alistar and was laning against a Tristana and multiple times i used the headbutt pulverise combo on her, both abilities would execute in the right order (headbutt then pulverise) but she would get headbutted away and not knocked up. (i know this sometimes happens when pulverise is used too late, but it seemed to be well in time). this happened a couple times during the game as well as another weird interaction where i would headbutt her, travel to her but not knock her back. (this happened once during the wind up for tristanas jump but i assumed that was just poor timing on my part). Should also add that tristana was laning with a morgana and i made sure to look out for the black shield to see if that was the reason, however it happened frequently when it had been used on other people. might be a bug with morgana