The site ID and allowed domains, the script.js and /api/v1/collect requests, exclusions, automation detection and blockers. Localhost is ignored unless enabled. See Install & verify.
Collect returns domain_not_allowed
The page's hostname isn't one of your site's domains or their subdomains. Add it in Settings → Site.
No collect request at all
Confirm the script tag still has data-site-id in the rendered page and isn't inside an iframe. Script optimizers can strip attributes.