Check the error in the synchronization's Details pane
If a synchronization has failed, in the WSUS administration console, go to the Synchronizations node, and in the middle pane select the failed synchronization. In the Synchronization Details pane you will see Details, which links to the full error description.
Synchronization retries by downstream servers
If the upstream WSUS server is not available for synchronization from a downstream server at the scheduled time, the downstream server will try to synchronize twice more, at approximately 15 minute intervals. If neither of the retries succeeds, the downstream server will try again the next day at the scheduled synchronization time.
Check proxy server settings by using the WSUS console
If your WSUS server is connected to Microsoft Update via a proxy server, you must use the WSUS console to configure WSUS so that it can access the Internet. For basic instructions about setting up a proxy server, see Deploying Microsoft Windows Server Update Services (http://go.microsoft.com/fwlink/?linkid=79983). If your proxy server supports authentication, make sure you have the correct user name, password, and domain. Note that if you use the WSUS console option for Allow basic authentication (password in cleartext), the password for the account is sent over the network in unencrypted text.
One major cause of synchronization failure is an expired password on the proxy server. Make sure the user name and password for the proxy server are always up to date.
Check the firewall settings
If your network has a firewall between the WSUS server and the Internet, make sure that all the necessary ports are open and the necessary domains are allowed. For more information, see Deploying Microsoft Windows Server Update Services (http://go.microsoft.com/fwlink/?linkid=79983).
Check the name of the upstream WSUS server
If your WSUS uses another WSUS server as its update source, make sure you are using the correct name for the upstream WSUS server and that you have spelled it correctly. For basic instruction about synchronizing two WSUS servers, see Deploying Microsoft Windows Server Update Services (http://go.microsoft.com/fwlink/?linkid=79983). The name that you enter in the WSUS console on the downstream WSUS server must match the name of the upstream WSUS server.
To determine if there is a problem with network name resolution services, use the ping command from the downstream WSUS server that cannot synchronize. You should use the same naming convention that is used in the WSUS console. For example, if you used a NetBIOS name in WSUS console, use the NetBIOS name of the upstream server with the ping command. If you cannot ping the upstream server, you might have a problem with network name resolution services. To work around this type of issue, you could use a different name resolution service or the IP address of the upstream server.
To contact an upstream WSUS server by using the ping command
Click Start, and then click Run.
In the Open box, type cmd, and then click OK.
Type the following, and then press ENTER:
where WSUSServerName is the name of the upstream WSUS server with which you are trying to synchronize.
Verify that users and the network service have Read permissions to the local update storage directory
If you store update files on your WSUS server, you need to ensure that the folder to which you download update files (by default C:\WSUS) has at least Read permissions for the network service and for users. This is true for both upstream and downstream WSUS servers.
On a downstream WSUS server, check that the updates are available on the upstream WSUS server
There are a number of situations where the updates on the upstream server no longer match the updates being requested at synchronization by the downstream server. Some of the following are examples of when this might occur:
An upstream WSUS server is reinstalled and the set of classifications and products the administrator selects is smaller than the set previously selected for the earlier installation. The downstream servers might then attempt to synchronize updates that the newly rebuilt upstream server has not downloaded. Synchronization will fail for updates that do not exist on the upstream server.
A downstream server is reconfigured to get updates from a different upstream server with different products and classifications selected.
To troubleshoot this issue, make a note of the updates for which download failed on the downstream server. These will be visible on the Updates page, and marked with a red "X." Check if these updates exist on the upstream server (look at the Updates page). If they do not match, do one of the following, depending on which updates you need:
Specify the missing updates on the upstream server, and then synchronize from the update source.
If the failed updates are not needed, cancel and then decline the updates that are not on the upstream server
If the missing updates are actually available on the upstream server, then the error is transient, meaning the update might have been downloaded to the upstream server after it was requested by the downstream server. This issue will resolve itself the next time the downstream server synchronizes to the upstream server.
Restart the BITS service
If the BITS service was disabled during synchronization, synchronization will fail. To ensure that the BITS services is properly enabled, restart both the BITS service and the WSUS service.
To restart the BITS service and the WSUS service
On the WSUS server, click Start, point to Administrative Tools, and then click Services.
Right-click Background Intelligent Transfer Service, click Properties, and make sure that Startup Type is Manual. After that click Start.
Right-click Windows Update Service, and then click Restart.
Retry synchronization: In the WSUS console, click Options, click Synchronization Options, and then under Tasks, click Synchronize now.
You should also ensure that both the BITS and the WSUS service are set to start automatically on reboot.
The number of updates that are approved on a parent upstream server does not match the number of approved updates on a replica server
This might occur if you have changed language settings on the parent upstream server after first synchronizing with the old language settings. For more information see "Listinactiveapprovals" in Managing WSUS 3.0 from the Command Line.
The last catalog synchronization failed
If your last catalog synchronization failed and you see event ID 10021 or 10022, check your upstream server and proxy settings in the WSUS administration console (Options, then Update Source and Proxy Server).
A WSUS 2.0 replica times out when synchronizing
In some cases WSUS 2.0 replica servers time out during synchronization. This issue has been fixed in WSUS 2.0 Service Pack 1 and in WSUS 3.0. See KB 910847, "Time-out error when approving multiple updates on Microsoft WSUS Server" (http://go.microsoft.com/fwlink/?LinkId=86496) for more information.