How do I turn on desktop notifications for my apps in Spanbox?

Two layers to check. First, allow notifications for Spanbox in your operating system (macOS System Settings > Notifications, or Windows notification settings). Second, make sure notifications are enabled inside each app too (Gmail's, Slack's, WhatsApp's own notification setting). With both on, new-message alerts come through and the unreads also stack in your unified inbox so nothing slips past.