@Patrick: Ok if you are sure that you have added in Featured image in all those posts.
Then can you go to “Appearance => Customize => Theme Options => Layout Options => Archive Content Layout” and check in your settings.
If this also doesn’t work then I need to check in your sever.
Why is my featured image not showing up in WordPress?
How to Fix Missing Post Thumbnails / Featured Image? If you do not see a featured image meta box in your post edit area, then click on Screen Options button on the top right corner of the screen and check the featured image check box. Now, you can click on ‘Set Featured Image’ link to upload your post thumbnail.
How do I enable featured images in WordPress?
To add a featured image in a WordPress post, simply click on “Set Featured Image” link inside the featured image meta box shown in the screenshot above. This will open the WordPress Media Uploader. You can use that to upload an image from your computer or use an existing image from your media library.
How do I make my featured image not appear?
To hide the featured image from the single post go to Admin Panel > Appearance > Customize > Theme Options > Layout Options > Image in Single Post/Page and chose the option No image from the drop down. It will disable the image from the single post .
What size should featured images be in WordPress?
1200 x 628 pixels”