Apple iPhone with Lightning notifications are missing

Model-specific

What is true of this model and not of its siblings

Apple iPhone XS Max notifications are delayed or missing

Notifications that do not arrive on an iPhone XS Max are usually set off in one of three places, and Apple's pages cover each. First the app: Settings, Notifications, pick the app, make sure Allow Notifications is on and that Lock Screen, Notification Center and Banners are ticked. Second, Focus: Do Not Disturb or another Focus silences alerts, so check Settings, Focus and the schedule. Third, the phone itself: the Silent switch silences sounds, and Low Power Mode and Background App Refresh can delay alerts for apps that need to fetch in the background. For Messages specifically, Apple has a guide for notifications that do not arrive. Check Notification Summary and Time Sensitive settings too. A restart refreshes the notification service, and signing out and in to the app can re-register it. If you restored from a backup, notifications must be re-allowed for some apps. An easy-to-miss cause is a connected Apple Watch or another device that takes the notification instead of the phone, because iOS shows alerts on the device you are using. Another is Screen Time's downtime, which blocks apps and their alerts during set hours. Check Notification Summary in Settings, Notifications, Scheduled Summary, because apps placed in the summary arrive only at set times, which looks like missing alerts.

Focus

A scheduled Focus is the most forgotten cause.

Messages

Apple has a dedicated guide for Messages alerts.

Other devices

If the phone is locked and an Apple Watch is on your wrist, the watch may take the alert.

Time Sensitive alerts

Some alerts break through a Focus only if the app is allowed in the Focus settings.

Summary

Alerts held for the summary arrive at set times.