Like an ad-blocker, Sticky Ducky does not leave placeholders or any other indication on the page; it just cleans up the clutter. The sign that it works is that the stickies fade away after a few scrolls. The extension also has uses heuristics in what sticky elements not to hide to avoid disrupting the page: the sidebars and full-screen splashes like a picture gallery are always shown.

It is very unusual to hear that you had so bad experience. I am always interested to know on which websites it doesn't hide stickies. The extension page links to the project github page where you can file an issue. Please include your browser version, the setting "When to show sticky elements" and links to the pages where the stickies remain.