Elder Dragon Not Executing Properly

Just had a very back and forth game, managed to wipe the enemy team, we take elder, and next fight, I snag the AD Carry as Thresh. I see his hp drop below the indicator 4 separate times, CLEARLY, yet no dragon proc for anyone on my team. First this executes at full HP, now it doesn't execute at all?!?!? Can someone please sort out the coding department at RIOT because nothing works these days. Just to be clear, we got other people low enough for the execute, but no "end game fantasy dragon breath" beam.

TL;DR Elder drag buff did not execute at all. despite multiple, CLEARLY VISIBLE, drops below threshold.