While playing as Xerath his Q - Arcanopulse, stops responding randomly. This usually occurs after a death and respawn, currently the only ways to fix this is to disconnect and reconnect or die and respawn. I play Xerath alot and it happens at least once every match.
Have you tried putting it on quickcast > quickcasting Q > taking it off quickcast?
It happens a lot to me with Vi's Q (haven't played her very much recently so I can't tell if it's still there or not) and that method has always fixed it for me in-game. Alternatively you could try self-casting it, haven't tried it myself though so no guarantees.
It's more a workaround than an actual fix though. I hope Riot looks into this, the bug's been here for years.
