I have just tried to switch from Summary (which was working) to Summary With Large Image. The validator says the cards are fine and I submitted for approval.
Here is a sample URL: http://lessonpix.com/materials/383707/initial+s+minimal+pairs+Puzzle+Cards
I got back an email saying:
“The URL you provided for lessonpix.com to use the summary_large_image card did not validate. Please make sure the page contains valid metadata, then reapply for a Twitter card.
Thanks for your patience. See you soon!”
But it does validate! The only thing that I changed was the card type (from summary to summary_large_image). And all the check-boxes in the validator are green.