Garen Decisive Strike Damage not affecting Nexus

Garen Decisive Strike Damage not affecting Nexus - [GAMEPLAY] I was playing on Summoner's Rift with some friends on purple/ red side. We pushed into their base and I was trying to get as much damage done onto the nexus before everyone died. When I went to use Q to reset my auto and do more damage, the animation to attack wasn't going off. I just stood there next to the nexus, and clicking on it made him walk into it or walk around it. And when the timer to use Q wore off, I was able to auto attack again. I try to do it again and it had the same results. Because of this, the game stalled for 5-10 more minutes and I lost the game.

I tried to reproduce the bug in a custom game and had no luck trying to get it to happen. If anyone else can get any information on this, it would be appreciated.