Siri doesn't appear to be functioning on macOS Sierra
Had the same issue and found out something that solved this for me. Apparently, my notification center process was disabled for some reason, I've enabled it via terminal by typing: 

launchctl load -w /System/Library/LaunchAgents/com.apple.notificationcenterui.plist
killall NotificationCenter 


After that I disabled and enabled Siri via the settings menu and it seems to be working now.