Twitter's validator doesn't seem to be able to access your site:
http://i.wpne.ws/QXQY
https://dev.twitter.com/docs/cards/validation/validator
That's probably what's causing the issue here.
If you don;t use any plugins that may block access to your site, I would suggest that you search the Twitter dev forums for problems similar to yours:
https://dev.twitter.com/discussions