I have this same problem. I think it might be because my subfolders aren't actually subfolders of the Inbox, rather they are at the same level as the Inbox (they were imported from Eudora).

Are you not receiving notifications for emails, calendars, and other events on the Outlook Web App (OWA)? Earlier, we covered solutions to get the notification working in Outlook’s desktop app. This ...

Is the Outlook (new) app not sending notifications on your desktop? Several users have reported that they are not receiving notifications for new emails, despite having enabled notification settings.