I have The Guardian app installed on my Android phone ( https://play.google.com/store/apps/details?id=com.guardian ), but although it is installed, the app card still says ‘Install Guardian app’ and links to the Play Store (see screencap: http://www.mkttl.co.uk/guardian_link.png - note this screencap is from the Beta programme, but it has been this way for some time, though it did work at some point). The Play Store link is correct and if you look at the source code in a Guardian article (example: http://www.theguardian.com/technology/2013/sep/13/best-android-apps-toy-story-xbox-music ), the tags appear to look as they should and when I ran that URL through the card validator, it validated okay. The YouTube links work, but I don’t see anything different in their app card tags that I would consider significant enough to cause an issue.
I have tried uninstalling / reinstalling / clearing app data on both the Twitter and Guardian apps and am now stuck!
Thanks for your help!