We'll need to look into this, send the video URL and screenshots. Also, any chance you have a YouTube Channel URL for one of your subs who didn't get the notification? A specific example would be really helpful to investigate. Which device your using to make a community post?
People that have enabled all notifications for your channel will be automatically notified. Live streams are also visible on home, suggested video, & the channel page. Your subs who aren't receiving live stream notifications can check their settings here: https://goo.gl/HzsDMW