I setup a WSUS server in a test lab and the results of client machines registering themselves with the WSUS server were flaky; especially if they weren't on the same domain as the WSUS server. One of my cloned machines was having trouble until I changed the SID with NewSID from Microsoft. The others didn't appear to have any duplicate SID problems but having them appear in the WSUS administration webpage was still a challenge.
Looking around the web, I found a variety of suggestions to fix the problem; most of them involved changing the registry. Here's a compiled list of those changes:
net stop wuauserv
REG DELETE "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate" /v AccountDomainSid /f
REG DELETE "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate" /v PingID /f
REG DELETE "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate" /v SusClientId /f
REG DELETE "HKLM\Software\Microsoft\Windows\CurrentVersion\WindowsUpdate\Auto Update" /v LastWaitTimeout /f
REG DELETE "HKLM\Software\Microsoft\Windows\CurrentVersion\WindowsUpdate\Auto Update" /v DetectionStartTime /f
REG DELETE "HKLM\Software\Microsoft\Windows\CurrentVersion\WindowsUpdate\Auto Update" /v NextDetectionTime /f
REG DELETE "HKLM\Software\Microsoft\Windows\CurrentVersion\WindowsUpdate\Auto Update" /v AUState /f
net start wuauserv
That looked very promising, but it didn't solve my problem. I was so frustrated until I found this command: wuauclt /resetauthorization /detectnow. Running that on the clients that failed to show up in WSUS worked every single time and it even worked on computers that weren't on the same domain as the WSUS server. Awesome.