Sober Time
First check that the notification is turned on in Settings › Notifications. If Sober Time isn't allowed to send notifications, a warning appears at the bottom of that page. Tap FIX to open your device's settings and allow them.
On Android, you can also turn off single notification types in the system settings. Sober Time shows a warning icon next to any type that's off there.
Battery savers and Focus or Do Not Disturb modes can also hide or delay notifications.