Tristana Ult not doing any damage [GAMEPLAY]

Hello Riot :)

So I was playing a ranked game recently, my last Tristana game, and this happened twice, but I can't quite explain why. But the first I remember very well. Caitlyn was very low on health, and I decided to tower dive her to kill her and get my reset to jump back out. I jumped in ulted, and then she got pushed back and away from me, but the ult didn't do any actual damage to her. I proceeded to follow her to get the kill but I never got the reset so the tower killed me. This happened a second time in the game, and it was less pivotal, and I am a bit fuzzy on it, I just remember it being twice. The first was very significant haha. Anyway, I don't know the consistency for it, but that is how it happened the first time.

Please and thank you for reading. :)