Android
Turn Lock Screen Notifications On or Off auf Android
Lock screen notifications are controlled separately from your regular notification settings. Even if an app is allowed to send you notifications, Android decides on its own whether that alert also shows up while the phone is locked.
This split exists because a locked screen is visible to anyone nearby, not just you. Turning the setting off keeps notifications from appearing until you unlock the phone. Turning it on but hiding the content shows that something arrived, without revealing the message text, sender or app details.
To change this behavior, open Tap on Settings ➔ Tap on Notifications ➔ Tap on Notifications on lock screen ➔ Enable or disable Notifications on lock screen.
If you only want to hide sensitive content while still seeing that a notification exists, look for the content visibility option in the same screen instead of disabling notifications completely. This is usually the better choice if you rely on glancing at your lock screen for quick updates but do not want message text exposed.
If notifications still fail to appear after enabling this setting, check the individual app's notification permissions as well, since a blocked app will never show alerts anywhere, lock screen included.