Darius GA Bug?

Just played a game with Darius and during a team fight my GA didn't pop. I had just bought it when I revived and we fought in our base and I just died, then it happened again next time in a team fight. The only time it proc'd was the third time after i had bought the item. My build (if it's even relevant) was merc treads with homeguard, black cleaver, hydra, randuins, GA and a null magic mantle at the time i had died the third time and it proc'd, the other times i just didn't have the null magic mantle. I just tested it again in a 55 minute AI game trying to re-enact everything from build to stacks on dragon buff but it functioned perfectly fine. Although right after i tested it i played another game on my 2nd account as Darius and it happened to me once out of 4 times again, the only difference was i had tabi's instead of merc's. It clearly isn't an extremely reoccurring problem, but its happened enough for it to be annoying, please help!