Image styles not creating files I am seeing a problem on my Drupal 7 site relating to image styles. 

A bit of background - This site was built by migrating the content and config from a previous site, using the Migrate and Features modules, after the previous site fell victim to Drupalgeddon. The new site seems now to be 99% working. (After fixing a lot of stuff that wasn't properly copied across by Migrate and Features - whether that was due to my inexperience in using these modules, or shortcomings of the modules, I don't know.)

One thing that isn't working is uploading image files into fields of type Image.

The Image gets uploaded successfully and is accessible from /sites/default/files/.
But no files are being created in any of the subfolders below /sites/default/files/styles. So the thumbnails don't display.

I've tried a lot of the advice I've found while Googling - the UNIX file permissions seem to be OK, the Drupal permissions seem to be correct, I can't seen anything wrong in the .htaccess files, but perhaps someone out there can give me better guidance on what to look for here.
Have you tried deleting the styles folder? The folder and everything below it should be created automatically by Drupal.