Hi Andy, with me happened something like that, I made what I found at Twitter and the post does not work.
The situation was this one: I looked for a solution to Twitter Card Summary, but I was looking for to do once not with all post, and I read that doing once and putting the code and doing the Card Summary validation and receiving the approval after this the action was not necessary for all posts, including old post supposed to work, this does not happens. So what I did? I put the instructions in the specific post and then I made the validation after this I got the summary.
Sorry for my bad English, I’m not American I’m Brazilian.
Thanks for your time and attention
Regards