Smush it PRO hangs for 12 hours

I've activated the WP smush it PRO plugin but it keeps hanging on for over 12 hours now. It doesn't do anything anymore. I think something is broke.

It says: 487 of 511 attachments have been sent for smushing and the button says smushing in progress. New images which I manually send are being smushed within a minute. All my older images which is sent 12 hours ago still show as 'currently smushing' in my WordPress library.