Perhaps specific to Twitter onebox then?
If I may: what say we take a look at this?
iFramely says there’s oEmbed data, but it’s not oneboxing.
ETA2: One more?
Make sure they have a valid text description, also some hosts will blacklist all crawl attempts from user-agents they don’t like. We see this a lot on WordPress installs hosted by WP-Engine, for example.
Since a couple of days, maybe a week, links to The Register won’t onebox anymore. They used to.
Archive.org’s radio plays seem to be clogging something:
In the preview, there’s a too large image, but nothing seems to show up after posting.
I pushed a fix for this:
… but the onebox looks awful as it’s just a tiny audio player that is taking up a lot of space because it’s embedded via iframe and the CSS is not optimized. The same can be seen on Twitter Card validator:
Will look for a workaround and then bump onebox version.