Start with Me › Reminders › Device status. It lists every setting that affects whether reminders arrive, and flags any that need attention. The most common cause is notifications not being allowed: the reminder is still recorded under Recent reminders, but no notification appears.
There are two kinds of reminders, and they differ in how punctual they can be:
Scheduled reminders (such as every day at 8 a.m.) are delivered by the system, even if Numable isn't open. On Android they may arrive a few minutes late.
Reminders that check data (such as a price falling below a level) have to get data first to know whether to notify you, so they run when the system gives Numable a chance: always when you open the app, and more often if you have a home-screen widget. On Free they check at most once every 30 minutes; with Pro they keep up with your widgets.
Platform notes:
iPhone / iPad: turn on Background App Refresh in system Settings, and don't swipe Numable away from the app switcher — after that, checks only happen when a home-screen widget refreshes.
Android: let Numable run in the background (see "Android: keep widgets updating on time"). To check right on time, turn on Live monitoring under Device status. It's a Pro feature; it keeps a notification in the status bar and uses more battery.
HarmonyOS: reminders that check data are mainly checked when you open the app or a service widget refreshes. Without System scheduled reminders, scheduled reminders are delivered only when you open the app or a service widget refreshes, so they may be late. Device status shows whether it's available.
Mac / Windows: Numable needs to keep running. By default, closing the window keeps it running in the background (in the menu bar on Mac, in the system tray on Windows). If you turned on Quit on window close in Me, closing the window quits it. Once it has quit, reminders that check data won't go off.