Hi, I had the same problem – thumbnails and other image sizes not being created.
Found the solution for me here: https://wordpress.stackexchange.com/questions/85803/no-thumbnails-generated
Basically, I didn’t have the php_gd extension enabled.
For me, running a linux on amazon EC2 instance, I just had to run
sudo yum install php-gd
sudo service httpd restart
and all my problems were fixed. Hope this helps someone.
It would be useful if the php extensions and wordpress dependencies were listed, perhaps on this page https://www.ads-software.com/about/requirements/