No Thumbnails

Hi
I copied my /uploads/ folder from OLDserver to NEWserver. I then ran the EXPORT on OLDserver and ran the IMPORT on NEWserver. It said that it worked, but the medi library showed empty thumbnails. Is there a step that I missed or does it take time to update the thumbnails in the background?
Here is a screenshot: https://imagebin.ca/v/5hN9OMeIMZ7o

I can see that lots of image rows have been added to wp_posts.

I have also checked and the URL for these image records are valid and correct.

What did I do wrong? Thanks!