[WP Smush Pro] WP Smush Pro not auto-smushing on upload

Both on our testing environment and our live, production environment (both set up as large multi-sites), we have Smush Pro set up to auto-smush network-wide (across all subsites) whenever a new image is uploaded to anyone's subsite media folder. The auto-smushing is not happening though.

When an image is uploaded, it is not smushed. When one goes into the media folder and opens the newly uploaded image it appears as if it is smushing, with the text "Smushing in progress" appearing. But after a short moment "Not Processed" appears, followed by a button that says "Smush".

If one clicks on the "Smush" button at that time, the image then gets smushed.

We need to find out why images aren't auto-smushing. We are using WP Smush Pro Version 2.9.1 on WordPress 4.9.8. We would be glad to give you access to our testing environment if if would help.

Thank you.