The issue
Sometimes you will send some images for optimization from the Media Library and while waiting for them to be optimized, it will seem that the plugin is stuck and not responding, for example here:
The explanation
If the Media Library is open in multiple browser tabs (for example, if more than one editor is working on your website), the ShortPixel background process is loaded in all of them. At the same time, it can only run in one window at a time (to avoid messing up the processing).
If you send a new image for optimization in one tab and another image in another tab, the images will be optimized, but the results will be returned to the tab that first requested them, which is not always the one that started the request.
The solution
Simply refresh your page to see the updated optimization status.