Vel'god Ult bug

I have been playing a lot of Vel'koz lately and in I remember since patch 5.21 or even 5.20 there was a bug on his ult where I will cast the ult but no beam comes out no damage is applied and after the 2.5 secs are done or if I press R again my ult goes on cool-down. Since it happened once I thought to myself well maybe I did something wrong or I'm just tripping balls or cancelled it, but it happened 3 times to me in this patch 5.22. So i will please like you to look into that, thanks.