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
Could you check with you subs, is this happening for desktop, mobile or email notifications (or all of the above)? Also, is this happening with *all* notifications or just certain types? Appreciate the additional info!