I’m recently having this issue also on our Wordpress site. The images on the summary card and large image card don’t display either the intended image or the fallback image. This happened today on 2 out of 2 cards. Then about 3 hours later, I checked my feed and the images were displaying.
I’ve had the cards working perfectly for about 6 months and then within the last two weeks started seeing this issue. If I test my URLs in the validator there is no error message displayed, everything appears to be okay, but the image doesn’t display in the card preview either.
Even cards that had displayed as intended, if I retweet them now, don’t display the images properly.
It seems like there is a lag in getting the images loaded. Cards that were tweeted yesterday showed no image but today show the image. Also cards that were tweeted this morning showed no images for a couple hours and now are showing the image. Only one of my cards from today that posted 2 hours ago still shows no image.