I have been trying to use WP Smush Pro to get the image sizes down on one of my client sites.
At first I tried using it and leaving it for a while whilst I got on with a few other things, but it hadn't made an progress, then I looked into possible causes and disabled the caching plugin that I had on the site and tried running it again overnight, but still no progress made.
I've tried smushing just one image individually from the media library but that didn't seem to make any progress either. I use the plugin WP Media Folder from JoomUnited to allow clients to put media files into folders. Smush is seeing more images when I disable this plugin, but still isn't smushing the images either in bulk or individual mode.
I've enabled Support access for you with anticipation you will probably want it. It's a Multisite Install, but I only have Smush activated on the individual site in question (it's a multisite setup as the client wants to develop the social aspects of their site with projects they undertake).
I look forward to your response and advice.