WSUS fail to downloading update

Hi,

I have a Windows Server 2012 with WSUS role has installed. 

I have already set the WSUS with the following configurations :

1. Set Update Source to "Synchronize from Microsoft Update"

2. Set Proxy server address with a credential user to connect to the proxy server

3. Specifying Product and Classifications

4. Set Automatic Approvals for the definition update

5. Set Synchronization Schedule to Automatically

But I cannot download the update files with the following error message :

"The files for this update failed to download. The update can be approved but will not be available to computers until the download is complete. Click Retry Download to start the download again."

I have click Retry Download, but download updates is always fail.

How can I trace the cause of the problem?

Thanks.

Regards, Bar Waelah