Delayed or missing notifications on the Galaxy S21 are a settings story, and the S21 is old enough that Samsung's battery optimization has had years to accumulate rules. The S21 runs One UI with Adaptive battery and an app sleeping list, and apps that are put to sleep stop pushing alerts. Open Settings, Battery and device care, Battery, Background usage limits and remove the app from Sleeping apps and Deep sleeping apps, then go to Settings, Apps, the app, Battery and choose Unrestricted.
The checks in more detail
In the app's own settings check that its notification categories are on. Do Not Disturb, Focus mode and a Bedtime mode silence alerts on a schedule. A VPN or a data saver can delay push messages over mobile data, so allow the app in Data saver. A Samsung phone of this age may have an old SIM or a carrier setting that affects messaging, so try the alerts on Wi-Fi to compare. If notifications come only when you open the app, background data is restricted.
Restart the phone, and reinstall the app if the problem is limited to one. Because the S21's battery has aged, Power saving mode may be on more than you think and restricts background activity. Check the Notification history as well, which you can turn on in Settings, Notifications, Advanced settings, Notification history, and which shows alerts the phone received but you did not see; if an alert is in the history, the phone received it and the delay was in showing it, which points at a display setting like Do Not Disturb.
This test shortens troubleshooting. Samsung's One UI lets you set notification categories and the badge style per app, and some owners find that App icon badges are off, which makes missed alerts hard to see. Turn on badges in Settings, Notifications, App icon badges so you can see unread counts on the Home screen, which helps tell a missed alert from a missing one.
Differential
Likely causes of “Samsung Galaxy S21 notifications are delayed or missing”, and how to tell them apart
- 1
The app is in Sleeping or Deep sleeping apps
What separates it Alerts arrive only when you open the app
How to confirm Remove it from the sleeping lists
- 2
Battery set to Optimized or Restricted
What separates it Alerts are late
How to confirm Settings, Apps, the app, Battery: Unrestricted
- 3
Do Not Disturb, Focus or Bedtime mode
What separates it Alerts silenced at set times
How to confirm Turn off or allow the app
- 4
Data saver or a VPN
What separates it Alerts delayed on mobile data
How to confirm Allow the app in Data saver; test with the VPN off
- 5
Power saving mode on
What separates it All background activity limited
How to confirm Turn off Power saving or exempt the app
Before you touch a setting
Record this first — changing settings destroys the evidence
- Which apps are late
- Whether the app is in a sleeping list
- Whether Power saving or Data saver is on
- Whether alerts are late on Wi-Fi too
Try in this order
0 of 8 completedOpen Background usage limits
ReversibleSettings, Battery and device care, Battery.
Remove the app from sleeping lists
ReversibleBoth Sleeping and Deep sleeping apps.
Set the app's battery to Unrestricted
ReversibleIn its App info.
Check Do Not Disturb and Bedtime mode
ReversibleIn Settings.
Check Data saver
ReversibleAllow the app.
Restart the phone
ReversibleThen test.
Turn on Notification history
ReversibleSettings, Notifications, Advanced settings.
Compare the history with what you saw
ReversibleTo see whether the alert arrived.
Model-specific
What is true of this model and not of its siblings
- An older phone
- Years of rules build up.
- Two lists
- Sleeping and Deep sleeping are different.
- History
- It shows received alerts.
- Display settings
- A silenced alert is still in the history.
Stop boundary
When this stops being a self-service repair
- !If every app is late after the changes, contact Samsung
- !If one app fails, contact its developer
- !Check the carrier for message delays
Evidence ledger
Sources behind this answer
Exact questions
Common questions
Why are my S21 notifications late?+
The app is asleep or restricted.
What is Deep sleeping apps?+
A list of apps blocked from running.
Does a VPN delay alerts?+
It can.
Will Unrestricted drain battery?+
A little.
How can I tell if an alert arrived?+
Check Notification history.
Where is it?+
In Notification settings.