fields with Image styles only showing placeholder
Here is an overview of how file permissions should be set on a Drupal installation: https://www.drupal.org/node/244924

If you scroll down there is a shell script that you can install on your server and it will fix all file/folder permission errors.

Your issue is definitely a folder permission error, Drupal has to be able to write to the /files folder. It makes new directories for each style, each image, each size so one folder may exist but the others may not be able to be made.