When a review reply is updated, I'm able to receive push notification. But where as for new question, update question and answer to a question is not notified via push notification. I subscribed to all changes.
URL: https://mybusiness.googleapis.com/v4/accounts/{accountno}/notifications
Output: 
I have given required permissions as well in pubsub settings
This is a bug.
I have reported this to the GMB API support multiple times - the new Notifications API has the same bug. The four Q&A NotificationTypes (NEW_QUESTION, UPDATED_QUESTION, NEW_ANSWER, UPDATED_ANSWER) can be set, however GMB never sends Pub/Sub messages for them.
You can let them know here: https://support.google.com/business/contact/api_default?hl=en
And hope that they create an issue here at some point: https://issuetracker.google.com/issues?q=componentid:585921%20status:open
