Why is a 200 instead of a 304 response returned?

I am testing caching with my nginx setup for a WordPress install. I believe I have things set up correctly, but I’m still seeing 200 responses instead of 304 responses for static assets. For example: curl -I –header ‘If-Modified-Since: Mon, Nov 25 2019 13:31:10 GMT-0500’ –insecure https://testdomain.com/wp-content/plugins/woocommerce/assets/js/jquery-flot/jquery.flot.time.min.js HTTP/1.1 200 OK Server: nginx/1.14.0 (Ubuntu) Date: Mon,…