[GAMEPLAY] Several champion ults disabling

This has happened many times over the last couple of months. Late game, I hit my R and...nothing happens. Clicking the icon doesn't do anything either. It happened notably last ARAM game with Kennen (who is nearly useless when he rushes in and taps with no ult) it lasts for about 4-5 minutes, then it's fine again (I ulted randomly while I was spamming R later). Has this happened to anyone else, and does Riot know about it?