When I run a URL through the validator, the image that I want to be displayed seems to be detected, but it is not the image displayed.

Eg, when the validator detects the following meta for one of my web pages:

The card preview instead displays my fallback image. A tweet for the same web page also displays the fallback image. Why is the image used in twitter:image not being picked up?

Could it be because the site’s robots.txt is blocking twitter?

Do you have an example URL? Maybe the og image tags are somehow coming into play.

I would also try using twitter:image:src as the tag name.