Vel'koz Ultimate Bug

I was playing a normal game on Summoner's Rift as Vel'koz middle and after a team fight around level 14 or 15 I used my ultimate. I died in the fight and respawned. I engaged another team fight later on, still level 14-15 and I was unable to manually click and hotkey r to use my ultimate. There was no way to reverse the move as I tried to go into settings and reset the keys. I ended the game max level, still unable to use the ultimate. Possible triggers: It could have been caused by a Gnar ultimate, Vayne condemn, a Thresh Q/E or a Syndra E while using the ultimate. Thank you!