typing [displayGallery id=5} doesn't display a result in a frontend, although everything is correct.
- is the gallery active
- does it contain images?
- is your shortcut written in a source code like <p>[displayGallery id=5]</p>
- did you refresh the page deeply with Ctrl+F5
if everything above is correct, please try to go to the gallery and resave it. Simply open the desciption and click the save button again.
Than please refresh your display page.