I´ve had this problem for a while and it seemed to be solved when I allowed the Twitterbot in the robots.txt file.
But now it happens again for most blog posts and for some not.
For example the image shows up for https://www.ecrowdinvest.com/blog/energia-sostenible-que-significa/ (image dimension: 1054x635).
But not for https://www.ecrowdinvest.com/blog/la-revolucion-de-las-energias-renovables/
(image dimendion: 1024x684)
The Twitter Card Validator gives no error message.
What could be wrong?