WhatsApp for Samsung Galaxy Core Plus

The Galaxy Core Plus is an older, low-RAM phone, so WhatsApp feels heavy. Clear WhatsApp's cache via Settings > Application manager > WhatsApp > Clear cache (never Clear data, which wipes your local chats). Free up internal storage so you have at least a few hundred MB spare, since a near-full device makes WhatsApp sluggish. Close other running apps from the recent-apps/task button, and keep WhatsApp updated from the Play Store. If it is still very slow, a current WhatsApp build may simply be near the limits of this hardware. See how others solved this on Reddit

Samsung's power saving stops background apps from syncing. Turn off Power saving mode under Settings > Power saving (or pull down the notification shade and disable the Power saving toggle). Then make sure background data is allowed under Settings > Data usage, and that WhatsApp notifications are on in WhatsApp > Settings > Notifications. Finally check that Do Not Disturb / blocking mode isn't silencing messages. See how others solved this on Reddit

First confirm WhatsApp itself is allowed to show notifications: WhatsApp > Settings > Notifications, and make sure message and group notification tones are set rather than None. Then force-stop the app via Settings > Application manager > WhatsApp > Force stop and reopen it, which re-registers it for push messages. If notifications still don't arrive, reboot the phone once so the messaging service re-connects. See how others solved this on Reddit

This is almost always power saving putting WhatsApp to sleep while the screen is off. Turn off Power saving mode under Settings > Power saving, and make sure background/auto-sync data is enabled under Settings > Data usage (and that mobile data isn't restricted for WhatsApp). With background data allowed and power saving off, WhatsApp can receive messages without you opening it. See how others solved this on Reddit

On an older phone, a weak signal is a common cause: the Core Plus on a poor Wi-Fi or mobile connection holds push messages until you wake it, so test on a stronger connection. Also turn off any data-restriction setting under Settings > Data usage > Restrict background data, and make sure WhatsApp's notification tone is set (not None) under WhatsApp > Settings > Notifications so alerts come through immediately. See how others solved this on Reddit

After a restart Samsung sometimes re-enables Power saving mode, so check Settings > Power saving and turn it off again. Confirm background data is still allowed under Settings > Data usage. As a clean reset, clear the app cache via Settings > Application manager > WhatsApp > Clear cache (not Clear data), then open WhatsApp so it re-registers for notifications. See how others solved this on Reddit