I wonder if this is the same issue I'm having with Hangouts and SMS notifications. It seems that if I have enough going on in the background, Hangouts will stop giving me notifications for SMS. When I go into the app, the new message(s) is there and waiting for me to read it. This to me is a huge problem. I didn't have this issue on CM11. For me, it seems it most always happens if I'm using the Facebook app and viewing a link from Facebook on Chrome. I've now made a habit of checking Hangouts periodically to see if I have any new messages.