Missing notifications can be caused by the individual app, Android notification permission, Do Not Disturb or device-specific battery management.
Check the app notification permission
Open the app’s information/settings screen and confirm its notifications are allowed. Some apps have separate notification categories that can be enabled or disabled independently.
Review Do Not Disturb
If notifications appear silently or not at all during certain hours, check Do Not Disturb and any scheduled modes.
Check battery restrictions carefully
Some Android manufacturers restrict background activity aggressively. Use the phone’s normal battery settings for the affected app rather than installing third-party “booster” software.
Update and restart
If the problem began recently, install normal system/app updates and restart the phone before clearing app data or reinstalling.