I've used the "Add Twitter card meta data" feature in the social menu of WordPress SEO by Yoast for quite some time and it used to work flawlessly. However, lately the cards don't show the image thumbnails any more.
I've run multiple URLs through the Twitter card validator (https://dev.twitter.com/docs/cards/validation/validator) and it does show a thumbnail in the cards.
I haven't changed anything in the plugin's settings. I've also compared the source code of two articles (one whose Twitter card shows a thumbnail, one whose doesn't) and there is absolutely no difference in the meta data generated by the WordPress SEO by Yoast plugin.
Does anyone else have this problem? If so, I think this is most likely a problem caused by a policy change on Twitter's end.