I was not getting a notification when choosing WP-Optimize > Purge minify cache from the WordPress admin menu bar on just one of my sites. The cache did purge, but there was no status alert. I tracked it down to having disabled “Show compression meta-box on an image’s dashboard media page.” in the WP-Optimize > Image settings.
The problem is with that setting toggled off the JS console reports wpo_parse_json is undefined after purging the minify cache. I tracked that function’s declaration to wposmush.js, which doesn’t get loaded in the admin if the meta-boxes are disabled.
For now I turned the image compression meta-boxes back on and the status alert is appearing as expected, but it would be nice if that wasn’t necessary.