You are facing the following phenomenon:
Your WSUS shows you that your clients need updates and that there is proper contact between clients and WSUS but clients say that they don’t need any updates.
Check your \WSUS\WSUSContent volume. Maybe older update files left no space on device.
How to cleanup? Follow the rules: ;)
*Close any open WSUS consoles.
*Go to Administrative Tools – Services and STOP the Update Services service.
*In Windows Explorer browse to the WSUSContent folder (typically D:\WSUS\WSUSContent or C:\WSUS\WSUSContent)
*Delete ALL the files and folders in the WSUSContent folder.
*Go to Administrative Tools – Services and START the Update Services service.
*Open a command prompt and navigate to the folder: C:\Program Files\Update Services\Tools.
*Run the command WSUSUtil.exe RESET
Stolen from http://blogs.technet.com/b/gborger/archive/2009/02/27/what-to-do-when-your-wsuscontent-folder-grows-too-large.aspx
HINT: If you need free space, run the ‘Options/Cleanup’ stuff in reverse (checkboxes from down to top) and only ONE ceckbox for a run. Otherwise the process probably will crash at the first checkbox and never do the rest.