I've had similar troubles myself with this plugin, but have recently found the solution.
Perchance, check the comments of the page in question; you can see that the developer Zoltan, says exactly this:
"If you have Output caching enabled, the minification will happen only once in a while, not with each request."
So output caching is a monumental concern. Here are the steps I partook to get the plugin engaged:
- log into your environment portal and within the "Settings" app, then go to "System" -> "Performance" and make sure "Output Caching" is disabled.
- Make sure that all of your checkboxes are checked for the "HTML Minification" setting in this same window
- Then navigation to "System" -> "General" tab and clear your cache and restart your application.
- Then clear your browser's cache and history
Once these steps were taken I was able to see all of my HTML minified within the "view source."
It was quite beautiful.