GAMEPLAY
Just completed playing a match on ARAM that started at roughly 0300 PST on 8/7/2015. First game of the day. Kalista targeted me with her black spear while I was playing as Garen. Throughout the match, I was unable to get the ult to go off. I right clicked on enemy champions, I right clicked on open field, I right clicked on minions thoughout the match. I was unable to get the ultimate's effect to go off, merely having it time out and place me down. I do not know if it is limited to just me, meaning I don't know if I kept screwing up due to not playing with Kalista that often, or if it is a known issue/affecting multiple players.
I do not know if I can recreate the problem as I was playing on ARAM.
Have a great day and I hope it gets resolved soon.