Sometime SCCM returns error message when you try to download an update:
The first thing in the troubleshooting process is finding a log file named PatchDownloader.log. The log file location is not obvious, because it not stored in the standard Program Files\Configuration Manager\Log folder with all other SCCM logs.
Instead, the PatchDownloader.log is created on your local computer from witch you open the System Center 2012 Configuration Manager console. And the file location is %temp% folder.
%temp% folder itself is located within your user profile: C:\Users\<user_name>\AppData\Local\Temp.
If you open the System Center 2012 Configuration Manager console in a Remote Desktop session from a site server, then %temp% location will be a bit different: C:\Users\<user_name>\AppData\Local\Temp\2.
Once you find the PatchDownloader.log file, open it with the CMTrace tool and review it.
The most likely reason that one or more updates are blocked by your security scanner, for example Websense.
Solution is to download the required updates manually (see the PatchDownloader.log file for URL links to the updates) and then import them into SCCM from a local disk instead of from Microsoft Updates site.