"Infinite scroll" means that contents is added to a web page if the user scrolls down. That way the page becomes longer and longer. An example for such page can be found here: http://isotope.metafizzy.co/v1/demos/infinite-scroll.html
The Webtrekk Scroll Position Plugin does not work with such pages. The plugin measures the height of the page by the first loading of the page. As the user scrolls down and more contents is shown and the page becomes longer, this additional page height will not be considered by the plugin.