[GAMEPLAY] Xerath Ult Bug

UPDATE -It's apparently just me being foolish- You need to press R not left click (like any other spell) to fire the ult once channeled....

Hello, I've just played another game (aram) where xerath's ult fails to work. This has affected half a dozen or so games I've played with him in the past couple of months.

I enter the ult's channel making him stationary, try left clicking away and not a single shot fires off so then I proceed to move (or recast) to cancel triggering the 50% CD. I've made sure I'm not cancelling the channel prematurely and I can see the cursor change and a range indicator during the channel but when I click no shots are fired and I stay there (until I cancel or timeout).

The bizarre thing is, in the same game, after about 6 attempts at using his ult, on one occasion it did work as it should... but I was doing nothing noticeably different.

This is extremely frustrating and I hope it can be resolved...