We have set up twitter cards on a new project and, although the cards validate and the images are publicly accessible the image will not display in the tweet or validator.
I have ensured that there is no robots.txt preventing crawling, and checked that images can be retrieved with curl -A Twitterbot
Images are within the size limits for this card type, summary_large_image.
Could this be a whitelisting issue? The images are served from a dedicated image server on a separate subdomain. I cannot find any tool to whitelist a domain separate from where the cards are hosted.
The project itself is not yet live but i have set up a sample that demonstrates the issue here https://s3-eu-west-1.amazonaws.com/capublicshare/sharetest_luna.html