Why is my Buy Button embed code from Shopify not loading on my WordPress site?

A Shopify Buy Button fails to render on WordPress when there are missing script integration tags in your page header, or when active security/cache plugins block external Shopify JS files.

“The short version: this happens when a required script tag is missing from your header, or a caching plugin is quietly blocking Shopify’s external file. I’ll track down which one and have it loading within 24 hours for $99.”
— Jeff Bollettino, Managing Director, Ignite Conversions