A missing notification can mean the message never reached the app, the app did not request an alert or Android presented it quietly. Check those stages before turning every notification control on.
Confirm the underlying event
Open the app and see whether the expected message or event is there. If it is absent, investigate the account or connection first. Notification settings cannot display an event the app has not received.
Inspect the relevant app controls
Check Android's app notification setting and any categories the app exposes. Then review the app's own preferences for the type of event. Avoid enabling unrelated promotional categories while trying to restore direct messages.
Check presentation rules
A quiet mode, lock-screen choice or sound setting can change how an alert appears. Read the active mode and its exceptions. Use an ordinary test message at a suitable time, not an urgent or emergency event.
Keep the successful path narrow
Once the required alert works, leave unrelated choices alone. Record which setting mattered so you can understand the arrangement later. If the message reaches the app but no supported setting restores alerts, send the developer a report with versions and test results.