There are different reason for this issue.
- Small image size.
You need to use the images of minimum 200 x 200 pixel size to publish to facebook (preferred is 1500 x 1500).
If the image size is less than this value it won’t publish to facebook.
- Image link is not accessible.
In some case if you are importing the post using another plugin, the image link is not accessible at the time of publishing. In such a case you can edit the post and republish to facebook or use the scheduling option in the premium plugin with a delay so that it will try to publish after the delay and at that time the image link is accessible.