Hextech chests not dropping

Hi all. Me and a friend just recently played a game where we both got S+ grades at the end. I had not played nami since the crafting update, so I know I still could get a chest with her. I also had 2 chests available to get before the game began. I didn't get a chest at the end, however. Am I missing something here?

Post-game link: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2144945164/49997687?tab=overview