System (device-level) notifications need all of the following: the site served over HTTPS (or accessed locally for testing — plain, unsecured connections elsewhere are blocked); you must have actually granted notification permission when your browser asked (if you dismissed or denied that prompt, it won't ask again automatically — you'll need to re-enable it in your browser's own site settings); and, specifically on an iPhone or iPad using Safari, the site must be installed to your home screen and opened as a standalone app, since Safari on iOS doesn't support these notifications in a regular browser tab at all.