How to rebuild images using Imagemagick?
This issue sometimes occurs when the original files are too large, causing memory exhaustion on the server during thumbnail creation.  Since the thumbnail creation on an image style cache flush is generated in a serial queue, just one high res image can prevent all the other image style thumbnails from being generated.

Steps to check and attempt to resolve this include:


After immediately trying to reset/flush the image style cache, check your apache and php logs on the server itself (as the memory exhaustion errors don't typically appear in the Drupal watchdog logs).
If you in fact encounter the memory exhaustion error, try increasing the server's memory limit.  See https://stackoverflow.com/questions/3534274/php-memory-exhausted
If after bumping the memory limit you continue to encounter this error, spot check some of the original images- determine the actual pixel dimensions of each image as well as the pixel density and/or dpi.  If either is significantly large, you may want to download the original images, batch process/resave at a more reasonable size, and upload back to the server; then trigger the image style flush/thumbnail regeneration again.
If #3 resolves the issue, you may want to apply max width/height to the image upload field config itself to prevent content managers from uploading enormous images in the future too. Note: Depending on your settings, this approach may result in saving a downsized version of the original image, if the original image exceeds maximum dimensions; or result in a warning to the content manager prompting them to upload a smaller image.
In conjunction with the max width &amp; height options, you may also want to change the jpeg quality of the images generated for the image styles.  While signed in as an admin, go to Configuration > Media > Image Toolkit (/admin/config/media/image-toolkit)