Hi again Maecia,
I worked out what the problem was on our environment. A Sophos filter that our IT partner has monitoring all web traffic on our local environment was forcing the Ajax calls from the WordPress>ReSmush.it admin page to timeout after 60 seconds. So it was killing the jobs if it did not manage to get all thumbnails for an image processed within 60 seconds.
I found it recording 504 (Timeout while reading response from Server) errors in the Browser Debugger (F12) > Console after a short period of time.
When I run the “Optimize all pictures” button from a web browser that is off our normal LAN it continues to work through the queue of images as expected.
So “it’s not you, it’s me” ??
Thanks for the plug-in. Really impressed with what it does so far and will shortly start using on our Production environment.