Hi,
You can first try out the following:
First, quit the game and the launcher. Now go into Documents/Electronic Arts/The Sims 3 and look for a folder called "DCCache". Delete everything inside of that folder, but do not delete the folder itself. Run the launcher again and then go to the Downloads tab. Notice that items on the list will not have an installed icon. Install the items again as you normally would. Items that did not install before should now install properly.
It is also advised that you clear your cache files before relaunching the game. This is especially true when adding or removing custom content from the game. The following cache files can be found in Documents/Electronic Arts/The Sims 3:
CASPartCache.package
CompositorCache.package
ScriptCache.package
SimCompositorCache.package
Deleting them should force the game to reload the most current graphics and script data in a pristine state. It is perfectly safe to delete the cache files as the game will automatically regenerate them each time.
