Android
Turn On/Off Notifications for Individual Apps auf Android
Android manages notifications on two levels. There is a general switch for all notifications, and a separate control for every app installed on your phone.
When one app keeps sending alerts you do not want, or another app has gone silent while the rest of your notifications work fine, the per-app setting is almost always responsible. It overrides whatever you have configured globally, so switching notifications on or off system-wide will not fix a single misbehaving app.
To change this, open Settings ➔ Notifications ➔ App notifications. This list shows every installed app together with its current notification status.
Tap the app you want to adjust and turn its notifications on or off. Some apps also expose notification categories underneath this toggle, letting you allow certain alerts (like direct messages) while blocking others (like promotions) from the same app.
Turning off notifications this way is different from disabling an app or clearing its background permissions. The app keeps running normally; it simply stops showing anything in the notification shade or on the lock screen. If you still get alerts after switching an app off here, check whether a second app (for example a messaging client with its own in-app notification setting) is generating them instead.