I was using the free WP-Smushit plug-in and using the batch process to process 50 at a time. I just upgraded to the pro version. When I press "Bulk Smush now" button, it changes to "Smushing in Progress" but nothing happens. I let it run for an hour and the progress bar doesn't move.
I did change or add any plug-in from when installed the free version and then upgrade to the pro version. Smushing individual files in the media library does work.