Hi FHL-FHL,
Please check if reindex WSUS database on WSUS server and reset windows update components on WIN 7 clients could work. Also check if there are errors in clients' windows update log.
Reindex WSUS database:
https://gallery.technet.microsoft.com/scriptcenter/6f8cde49-5c52-4abd-9820-f1d270ddea61
Reset windows update components:
https://support.microsoft.com/en-us/kb/971058
After resetting windows update components, it may take some time for clients to report to the WSUS server, so, we'd wait for a period of time.
Best Regards,
Anne
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