[YASUO] Delayed ability to use ultimate

I've been playing nothing but Yasuo all day today, and I've owned the champion since shortly after his release. However, when a teammate knocks an enemy airborne, I attempt to cast Last Breath and sometimes, it won't let me actually cast the ability until just moments before the enemy touches the ground again. Sometimes it won't allow me to cast the spell at all. I mash the R button as quickly as i can, and it waits until almost the end of the window of time that i have to cast the ability. It JUST started doing this today after i repaired my client because another issue I'm having is that when I play Yasuo, the game crashes IMMEDIATELY after the loading screen, but I've reported that bug in a different thread here.

I have a pretty solid connection using an Ethernet cable directly plugged into our router with Time Warner Cable Turbo internet. With that being said, along with the fact that as this problem is happening everything else is normal, I don't thing lag is a factor. This champion is pretty bugged in my opinion, there are plenty of things that pass through wind wall that shouldn't, and it kind of upsets me because he's definitely my favorite AD mid laner.