I have gone through https://www.nextscripts.com/snap-features/how-snap-sets-images/ although it does not apply here. I have done test posts with no featured image and send them to Flipboard and the post and image show correctly on Flipboard.
I did some extensive testing over the last few days to get to the bottom of the problem and checked the flipboard profiles of others users and see the same thing happening.
From all my testing I can conclude that:
1) if the post is similar (in content) to another post on Flipboard, then Flipboard will not show the image
2) If you send a post to flipboard , which already exists on flipboard, then the second post will not show up
3) On a row (on flipboard) if one post is bad, then no images will show on other posts on the same row, even if the other posts have good images. This behavior is seen provided you are logged into flipboard.