Hextech Chest Not Dropping After S Game

Can someone help me out here? I have not gotten a hextech chest after my last several S games in which I complete a mission. I've gotten them from games in which no missions were completed, however. My theory is that when I complete a mission in the same game there is a bug that prevents the hextech chest from being rewarded, but I've submitted several reports and gotten no response from Riot either way. This just occurred again, and my account is showing 2 chests available. Am I missing something here? Are there restrictions beyond simply the 1 chest/week limit?