Today I had some servers which weren’t visible in WSUS. So I searched on the internet and found that the following steps will solve these problems:

1 . Delete the corresponding Reg Key’s:

HKLMSOFTWAREMicrosoftWindowsCurrentVersionWindowsUpdateAccountDomainSid
HKLMSOFTWAREMicrosoftWindowsCurrentVersionWindowsUpdatePingID
HKLMSOFTWAREMicrosoftWindowsCurrentVersionWindowsUpdateSusClientId

2. Restart the WSUS Client service:

net stop wuauserv
net start wuauserv

3.  Finally reset the WSUS settings and detect new updates:

wuauclt.exe /resetauthorization /detectnow