Problem: 
typing [displayGallery id=5} doesn't display a result in a frontend, although everything is correct. 

First aid:
  • 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
Solution
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.