It seems that Netbeans 8 has a pretty "dumb" way of looking for LESS files by default. That default is that Netbeans looks in the project root for a folder called "less" that it then monitors for changes and compiles the output into a folder called "css" (both in the root of the project).
When working on something like a Drupal or Wordpress project, you might have your content be logically saved (for some reason) in a folder like: wp-content/themes/my-theme/lib/less.
So, what you need to do is add and/or modify the input and output folders to make sure that you're achieving the desired results. From the Project Properties windows select "CSS Preprocessors" and from there you can choose to either modify the defaults, or you can add a new folder to the watch list and have it get compiled into any directory of your choosing. The Input and Output paths are relative to the document root so you might do something like: /lib/less for the input source and output directly to the /css folder or it could be something like /static-assets/styles/compiled/css (for all that Netbeans or anyone else cares).
It is rather strange that Netbeans doesn't automatically set the input folder to the folder where you created the LESS file but such is life, it's not the first time I've experience a "Huh?" moment when working with Netbeans.
