Looking at the two most recent posts on my test account here https://twitter.com/testingstuff244, the most recent preview image is a black placeholder image, whereas the second preview is properly previewing the correct image.
From what I can tell there isn’t any specific reason why the one would work and not the other. The card validator indicates everything is fine with both embeds and the meta tags for each are nearly the same.
What reasons might there be for the
twitter:image meta tag to be ignored or is there any way to determine why the image didn’t work for the one tweet?