You can try the below steps to debug and troubleshoot:
Personalization Issue: Missing user data can cause personalized content in notifications to fail, resulting in a blank notification. Make sure that the user's profile data and ensure all necessary fields are complete.
Payload Issue: The notification payload might be missing the title or body. Check the payload sent to the user to confirm it contains a valid title and message.
Device-Specific Issue: The user's device may have problems displaying notifications.(I saw an incident where the user was using very old Android version causing a similar issue )Advise the user to restart their device or clear the app cache.
OS or App Notification Settings: The user may have disabled notification previews or background notifications in their device settings. Guide the user to check the app's notification settings on iOS or Android.
Network Issues or VPN: A weak network or a VPN might be blocking push services. Have the user test the notification on a stable Wi-Fi or mobile network
