missing thumbnail images

I have some missing thumbnails. Is there a recommended manner to troubleshoot this? They all have a post image show up when you click on the name of the post.

Thanks.

strandbeest post and sweeps post here:
‘https://formpig.com/blog/category/geometry/’

the meaning of architecture post here:
‘https://formpig.com/blog/category/architecture/’

catastrophe theory post here:
‘https://formpig.com/blog/category/bibliography/’