Hello. My website is http://allthingssports.com/ and I have the same problem, Twitter Cards were validated two weeks ago and the validation checker also says they are working but twitter curds not showing all the time on Twitter.
I wonder if it’s the problem with my site or with the twitter cards service and you’ll fix it soon
An example page with the meta tags: http://allthingssports.com/mls-president-says-new-york-is-a-solid-soccer-market/
and a Tweet that should have the Twitter card: