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.