There are multiple reasons why Facebook is not displaying the correct thumbnail image in WordPress.
One of the most common reasons is that you have multiple images placed in the og:image tag, with your displayed image being smaller than the rest of the images. Facebook uses Open Graph (og) tags. Plugins like Yoast SEO will automatically add them to your site to avoid an image not showing up. Other reasons why Facebook is not displaying the correct featured image are cache plugins, CDN issues, and missing Open Graph meta tags for the thumbnail image.