If you are not seeing any thumbnail images being generated you should check your PHP version. There was a bug introduced in PHP 5.6.24 that causes the thumbnail process to fail for some images.
The fix is to roll back to 5.6.23 or forward to 7.0.x
For those interested the bugs are listed here: https://bugs.php.net/search.php?cmd=display&package_name[]=EXIF+related