My site, daffadillies.co.uk, is approved for twitter cards, and I run several different sites hosted on that domain, including two webcomics, Fetch and Eunice.
Photo cards seem to work perfectly fine when I link to a Fetch comic, but don’t appear when I link to a Eunice comic. The only difference I see is that when I run the validator tool, Eunice has a warning, “unknown aspect ratio,” while Fetch does not. The only thing that changes is the image URL, and there is no difference between the average aspect ratios of images on the two sites.
Any idea what could be causing this?