This has been the issue for our site as well; at Rewire we went from 4.5 million impressions in October, about average for us, to 600K in November, with no updates at all since November 4. We have had Twitter cards installed for the last year and the cards are actually appearing correctly in Twitter — here’s a typical log for one of our links from Twitter’s validator:
INFO: Page fetched successfully
INFO: 24 metatags were found
INFO: twitter:card = summary_large_image tag found
INFO: Card loaded successfully