If you haven't been reliably receiving notifications for messages in Google Voice, you may not be alone. Some users at Google's Product Forums have been reporting that notifications from Google Voice don't always appear, and often don't show up until the app is opened and refreshed. Thankfully it seems that Google is on the case. If indicators for events like messages or voicemails in Google Voice do not show up in your notifications, perhaps it's worth taking a look at the threads over on Google's forums.
I've actually been experiencing this problem, myself, but until I saw the forum posts, I had attributed the cause to Google Voice not working with Doze as it should. Most reports indicate that calls to Google Voice are still coming through properly, but notifications for voicemails passed to the service and messages received are not appearing on some user's devices. Some also claim that problem is also manifesting on pre-Marshmallow devices so it may not be attributed to Doze.
The Google Voice team has created a questionnaire for those with the problem so that they can better identify a common source of the issue. If you are among those experiencing the problem, consider taking a look at some of the forum posts in the links below to verify your symptoms before answering the questionnaire.