I bought and installed the Highlighter Pro plugin, installation went fine. But I can't get it to work on the website -- not on pages, posts, nowhere really. I changed z-index from 97 up to 101, no change.
It seems to load Highlighter Pro in source code of pages, but somehow nothing happens when I select text on the page.
It's possible the text you are selecting isn't part of the main site content. Do you use the main content of your posts/pages, or do you use special components like widgets, custom meta boxes, or other custom page builder elements added by plugins? Have you tested this yet with the default WordPress theme and no plugins activated, just to make sure there isn't an issue with your WP installation?