GA Not Activating

I just played a game as Hecarim, and during the game I bought the Guardian Angel item. About 2 minutes after I purchased it, there was a team fight and I was killed by an Ezreal q. I did not die at all since buying the GA, so the passive wasn't on cooldown or anything and it didn't activate. It's probably a really tiny bug, but I just wanted to share it :D. Thanks