How to debug WordPress site oneboxing?

[robmc (Robert McIntosh)]

This might help you (see ref WPEngine)

Why Do Some Blog Posts in Discourse Have A Preview?

Discourse oneboxes do work with WordPress sites. If a link fails to onebox, it is likely because either the page it links to is missing the necessary Open Graph data, or because the request from Discourse is being blocked by the website’s server. You can check the Open Graph data for a URL here: http://iframely.com/debug. The website you are trying to link to is hosted by WPEngine. They are blocking requests from the Discourse Forum Onebox user agent. To allow sites hosted by WPEngine to cre…