Why is my Klaviyo tracking code not identifying active on-site users?

Klaviyo fails to identify active users when the onsite JavaScript snippet (Klaviyo.js) is missing from your theme header, or when users visit your site without clicking an email or submitting a form.

“The usual culprit is a missing tracking snippet in your theme header, so Klaviyo never gets the chance to identify anyone. I’ll confirm it’s installed correctly within 24 hours for $99.”
— Jeff Bollettino, Managing Director, Ignite Conversions