A common issue where the favicon will not show up when expected is cache, if your .htaccess for example reads:
ExpiresByType image/x-icon "access plus 1 month"
Then simply add a random value to your favicon reference:
<link rel="shortcut icon" href="https://example.com/favicon.ico?r=31241" type="image/x-icon" />
Works every time for me even with heavy caching.