you might encounter such case where one of your wsus server clients are not getting updates, what you have to do is to go to the client computer and apply the below.
1- proxycfg -d to delete outbound proxy and forece wsus serve rdetection
2- reinstall windows update agent and check if there is registery an access denied on it
3- stop the windows update service and then rename the software distribution folder under c:\windows\system32
4- try detection again.