Reminder notifications not working on Android? How to fix it
Short answer: Most "my reminder didn't go off" problems come from four settings: the app's notification permission, the "Alarms & reminders" permission, battery optimization, and the sound setting of the notification channel. Check them in that order.
1. Notification permission
On Android 13 and later every app must ask before it can show notifications. If you tapped "Don't allow" once, reminders are created but never shown. Open Settings → Apps → your reminder app → Notifications and turn them on.
2. Alarms & reminders (exact alarms)
Android can batch alarms to save battery, so a 9:00 reminder may arrive at 9:07. Apps that need to fire on the minute ask for "Alarms & reminders". Find it under Settings → Apps → Special app access.
3. Battery optimization
Some phone makers stop background work aggressively. If reminders only show up when you open the app, set the app's battery usage to "Unrestricted".
4. Channel sound
Each notification category can have its own sound. If a reminder appears silently, open the app's notification settings and make sure the reminder channel is set to "Alert" with sound.
After a restart
Alarms are cleared when a phone restarts. A good reminder app re-schedules them automatically; if yours doesn't, open it once after rebooting.
In Pingday
Pingday uses exact alarms, turns sound and vibration on by default, re-schedules after a restart, and has a "Send a test reminder" button in Settings so you can check everything works in one tap.