I have my Google Voice account setup through the Hangouts app for Android, on my un-rooted Galaxy S5 (with the Lollipop OTA).
When I have the application open on my device, and I receive an SMS to my GV number, it appears on screen.
If I don't have the application open, the phone still receives the message, but it does not give me a notification. I know background data is turned on, and it has received the message, because I can turn on airplane mode, then open the app, and see the message is there unread.
SMS messages sent to my carrier number as well as Actual Hangouts messages notify correctly whether the app is open on-screen or not.
Has anyone ever seen this before?