iOS — Safari never shows an auto-banner. Use Share → Add to Home Screen. Chrome/Firefox on iOS can't install PWAs at all (Apple restriction).
Android Chrome — the banner appears after ~30 seconds of use. Force it via ⋮ menu → Install app.
Desktop Chrome/Edge — click the install icon in the address bar (right side).
Already dismissed the banner? It won't reappear for ~90 days. Use the browser menu instead.
Firefox / Brave — install support varies. Chrome/Edge/Safari-mobile are the reliable paths.