When playing an ability power champ which the build includes the item Zhonya's Hourglass, I come to find that in a teamfight the item decides to work when it wants to. It happens every time when I purchase this item. When out of combat and I hit the correct key it works 100% of the time, but when combat is involved the item decides not to work. My friends are also having the same problem so I know its not just me.
There's a problem I have too, I like to call it "Bad reaction time"
Usually this is because you are silenced or otherwise CC'd and can't use it when you try, not because of a bug. If you can't figure out what happened I recommend downloading your replay and rewatching it. You can also get the Overwolf app to watch the segment right after it happens, and see exactly when you were pressing your keys.
Maybe user error
You can't use Zhonya's to break CC. That is likely why it isn't working for you every time you press the button.
Try using it before you get stunlocked.
