Preview Image/Features Image is displayed only for half of my post

Dear Sir/Madam,
I am having an issue concerning the preview image or features image who should normally be displayed on my blog section. 2 out of 4 article is not displayed.

Blog URL: https://dedansphotography.com/en_GB/blog/
I am using photocrati template and here is what I’ve already done on the NEXTGENpro Plugin:
-I’ve been uploading new images and even created new thumbnail in the “Gallery Setting.” Sadly it still not working.

Could you kindly help me on this topic.

Look forward to your reply