When specifically does R grant a stack?
So, I've just got this champ for the first time in ARAM and was a bit frustrated with my ult never giving me stacks even though I hit multiple people and all of them died in the fight. I ended the game with 2 stacks on R even though it didn't even go poorly and we ended up winning. So does R really only give the stack if the enemy is killed specifically by the execute and nothing kills them over it, not even Locke's own damage? Or am I misunderstanding something else fundamentally about this ability. I even missed the stack when I hit R and then got the kill on the enemy in a 1v1 at least two times. I'm certain it wasn't because it was past the 5 seconds. I don't get it at all.
Edit: Ok, solved. I didn't realize I need to pick it up. My bad. /closed