Sometimes our WSUS failed to download certain updates. Lately the Malicious Software Removal tools and Outlook junk mail filters have been very problematic. The problem is somehow related to the BITS and its download mode. Solution for the problem is to download WSUS Debug Tool and set the download to the foreground mode.
Download the wsusdebug tool: http://download.microsoft.com/download/7/7/4/7745a34e-f563-443b-b4f8-3a289e995255/WSUS%20Server%20Debug%20Tool.EXE
Run command: wsusdebugtool /tool:setforegrounddownload