There are many reasons why it's needed, but sometimes a full reset of the WSUS is required. Though a time consuming process, a reset causes the WSUS server to resync with Microsoft services. The process is a long one since each update that resides on the WSUS server must be resynched, so don't be afraid when it takes longer than you think it should.
In the immortal words of Ace Ventura, Pet Detective: "If I'm not back in five minutes, just wait longer."
The reset will actually pull down all updates to ensure that your WSUS server is packed with everything that needs to be there to continue delivering updates to end-users computers. If for some reason files are missing, or were accidentally deleted, the reset will put them back.
In order to run Reset, run the following command as an administrator:
%SystemDrive%\Program Files\Update Services\Tools\WsusUtil.exe reset