WhatsApp for Samsung Galaxy J3

Samsung's aggressive battery management is usually the cause. Go to Settings > Apps > WhatsApp > Battery and set it to Unrestricted, then open Settings > Device care (or Device maintenance) > Battery, find WhatsApp under the app power management / sleeping apps list, and make sure it is not set to "Put app to sleep" or "Deep sleeping." Next open Settings > Apps > WhatsApp > Notifications and confirm notifications are allowed with sound and pop-up enabled. This keeps WhatsApp running in the background so messages arrive instantly. See how others solved this on Reddit

Start inside WhatsApp: tap the three-dot menu > Settings > Notifications and make sure each tone is set rather than "None." Then check the system level at Settings > Apps > WhatsApp > Notifications and confirm it is turned on. Make sure background data is allowed under Settings > Apps > WhatsApp > Mobile data (enable "Allow background data usage"), and that WhatsApp is not in the sleeping apps list under Settings > Device care > Battery. Restarting the phone afterward lets the notification service reconnect. See how others solved this on Reddit

WhatsApp needs permission to display over other apps and to run freely in the background. Go to Settings > Apps > WhatsApp > Permissions and grant the Phone permission, then enable "Appear on top" / "Display over other apps" (under Settings > Apps > Special access > Appear on top if it is not listed in the app's permissions). Next set the app's battery usage to Unrestricted and keep it out of the sleeping apps list so the call screen can wake the device. If calls still fail to appear, reinstalling WhatsApp from the Play Store re-registers the call service. See how others solved this on Reddit

The J3 has limited RAM, so clear WhatsApp's cache via Settings > Apps > WhatsApp > Storage > Clear cache, and delete old media-heavy chats to free space. Update the app through the Play Store, since outdated builds run poorly on legacy hardware, and keep WhatsApp off the sleeping apps list so Samsung's memory saver does not keep closing it. If it still struggles, lowering auto-download under WhatsApp Settings > Storage and data reduces the load on the device. See how others solved this on Reddit

Notifications only forward to a watch if they first appear on the phone, so on the J3 confirm WhatsApp notifications are enabled and that the app is not being put to sleep by battery management. Then open the Galaxy Wearable app, go to Watch settings > Notifications, and make sure WhatsApp is toggled on in the app list, including the "Notifications while using phone" option. If it still does not relay, toggle WhatsApp off and back on in Galaxy Wearable to re-sync. See how others solved this on Reddit

This almost always means a paired wearable is mirroring the alerts from your phone, or a device is linked through WhatsApp's Linked Devices feature. Open the Galaxy Wearable app and remove WhatsApp from its notification list so the watch stops mirroring. On your main phone, open WhatsApp > Settings > Linked Devices and log out any device you no longer use. If a phone is showing them, check Settings > Apps > Special access > Notification access and revoke access for any companion or relay app you don't recognize. See how others solved this on Reddit