Google Voice SMS not showing notifications in Hangouts? 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?
It looks like there is an issue with Hangouts app that causes notifications for new conversations with a given contact to be disabled by default, with no way to override this setting.  On the Conversations tab, these are denoted by a bell with a strike-through:

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Once you open such a conversation you will see a message in red saying "Notifications off - You won't be notified of new messages" with an option to enable them:

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Existing conversations for which you've enabled this setting will receive notifications normally.  If you delete the conversation, the setting will revert to "off".  Archiving the conversation keeps it enabled.

The details that are described in this thread (I suggest starring it to show Google that there is interest in this issue) match exactly what you are experiencing.  It has been brought up before, and was escalated to the Hangouts development team, who are already aware of the issue.  We can only guess when (or if) Google will fix this.