Flipboard won’t pull featured images

Hello,

Since the last plugin update, my featured pictured stopped appearing on Flipboard. I contacted
Flipboard and their answer was:

This looks like an issue of website optimization. Take a look at these RSS feed
optimization guidelines our team has put together
https://about.flipboard.com/rss-spec/

However, my RSS feed shows featured images, so I can’t figure out what the problem is. How can I make featured images from my posts appear on Flipboard again?

Thanks!
Elena