WhatsApp for Samsung Galaxy Note T879

Samsung's aggressive battery management is the usual culprit, since it puts WhatsApp to sleep in the background. Go to Settings > Apps > WhatsApp > Battery and set it to Unrestricted, then open Settings > Battery and device care > Battery > Background usage limits and make sure WhatsApp is not in the Sleeping or Deep sleeping apps list. Also confirm Settings > Apps > WhatsApp > Notifications is turned on. See how others solved this on Reddit

Software updates often reset notification channels and re-enable power saving for apps. Open WhatsApp > tap the three dots > Settings > Notifications and toggle the message and group notifications back on, then in phone Settings > Apps > WhatsApp > Notifications re-check that every category (Messages, Group notifications, Calls) is allowed. If it still fails, clear the WhatsApp cache via Settings > Apps > WhatsApp > Storage > Clear cache and reboot. See how others solved this on Reddit

Duplicate alerts usually mean WhatsApp is active on both your phone and a linked device (like Web/Desktop or a watch), so both surfaces alert. Open WhatsApp > three dots > Linked devices and log out of anything you are not actively using. If alerts still double, force stop WhatsApp via Settings > Apps > WhatsApp > Force stop, then reopen it. Clearing the WhatsApp cache under Settings > Apps > WhatsApp > Storage can also clear a stuck notification state. See how others solved this on Reddit

Delayed delivery is almost always background data or Adaptive battery throttling WhatsApp. Go to Settings > Apps > WhatsApp > Mobile data and make sure Allow background data usage and Allow data usage while Data saver is on are both enabled, then turn off Adaptive battery under Settings > Battery and device care > Battery > More battery settings. Keeping WhatsApp's battery setting on Unrestricted (Settings > Apps > WhatsApp > Battery) keeps the push service alive. See how others solved this on Reddit

A WhatsApp or One UI update can reset notification channels to a default sound and behavior. Open WhatsApp > three dots > Settings > Notifications to set your custom tone and vibration, then in Settings > Apps > WhatsApp > Notifications > Messages set the importance to Alert (not Silent) and enable Pop-up display to bring back the floating banner. Turning on Notification history in phone settings helps you confirm alerts are actually firing. See how others solved this on Reddit

Watch notifications only mirror what the phone shows, so first confirm WhatsApp alerts work on the phone itself. Then open the Galaxy Wearable app > Watch settings > Notifications and make sure WhatsApp is toggled on, and turn off 'Show notifications only while wearing watch' if the watch isn't detecting your wrist. Also check that your phone isn't restricting WhatsApp's background activity, since a sleeping app won't forward alerts to the watch. See how others solved this on Reddit