I have noticed while trying to post to twitter that robots.txt would block images from being displayed resulting in a tweet without an image.
It goes against the great care you took to include all the required informations for “Twitter Cards” to work properly.
As recommended on twitter’s developper’s pages your robots.txt should be similar to the following:
User-Agent: * Disallow: /a/*.json Disallow: /a/*.png Disallow: /a/*.gif User-agent: Twitterbot Allow: /a/*.png Allow: /a/*.gif
This is keeping the currently configured robots and adding the Twitterbot’s user-agent