Poppy Ult dissappears

I played a 5v5 Summoner's Rift, draft pick game.

I was playing as poppy. Throughout the game, the ultimate worked perfectly fine.

However, towards the end of my game, my ult decided not to work and this cost us our victory.

-The Ultimate was used on sejuani -cooldown went off but the ultimate did not

Please look into this situation.