Some creations may have specific install instructions, did you read the requirements from the download?
Some creations need a mesh from another creator, which probably should be be downloaded from the creators site in order to get a creation to work in game.
Did you read the requirements if a mesh is needed for this download?
Do you have the expensionpack which is needed for the specific download, if an expansionpack is needed it is mentioned in the desription area from the download, see requirements.
Did you update your game?