Sometimes, for various reasons, one must perform a WSUS reset function to make the WSUS server happy again. The reset process forces a resync of each update in the WSUS server with Microsoft services. This task can take a long time to complete as each update will be re-evaluated and re-downloaded, so be patient.
Here’s the command to run from an administrator command prompt::
%SystemDrive%\Program Files\Update Services\Tools\WsusUtil.exe reset