[GAMEPLAY] Morgana Ultimate not triggering

In a Ranked game I just completed as Morgana, there were multiple occasions where my Ultimate was definitely in range (champs were very close and icon changed to lit) but pressing R did nothing. The first time or two it happened very quickly before I died so I wasn't sure if I got it off, but in the last fight I was in I had 3 champs around me for a very long time and repeatedly pressed R (5+ times) and it never went off. I immediately opened chat to verify that the R key on my keyboard was functioning and all keypresses registered.