Blog thumbnail not showing on frontpage

Hi,

I can not figure out why my blog post images are not showing correctly as thumbnails on the frontpage/homepage. The first post I created is working correctly with the thumbnail showing, however, any blog post I have tried creating since is just showing a white square where the thumbnail image should be. See here:

https://northshorechiro.co.nz/

Please help! I can’t see why one is working but not the other?!