I have noticed very few sites I attempt to use a Onebox for are not working. I checked
https://blog.scoutingmagazine.org/2016/11/03/the-boy-scout-board-of-review-everything-you-need-to-know/
against iframely.com and it displays as expected. iframely.com/debug does not indicate any errors.
Is there something else I can check?
My guess is you are having your crawl rejected (and this is happening here on meta too).
But I have a solution, but it’s not 100% free:
I just tested this on two of my sites, one equipped with the plugin, and one without, and it works on the one equipped.
My pleasure! 
One of the issues here is that Discourse/Onebox could be a heck of a lot more transparent about the way it handles a failure. I guess that’s a stretch feature, but it would be nice if there was more feedback to the user pasting the link (or even just the admins).
I checked my /logs on the site which failed and do not see any extra information wrt to the failed preview …
This is funny, that site blocks “forum” in the User Agent:
Works:
Don’t Work:
Yes, I can reproduce that from a VPS using curl -A "-string-"! Can we have a setting to change the user agent? 
In this case that would be a lot less hassle 
Oh wait, I see a file:
discourse/config/initializers/100-onebox_options.rb
require_dependency 'twitter_api'
Onebox.options = {
twitter_client: TwitterApi,
redirect_limit: 3,
user_agent: "Discourse Forum Onebox v#{Discourse::VERSION::STRING}"
}
Take the word forum out?
I could easily add this to my plugin of course …