If you are seeing a group of clients whose patch status is unknown it would be helpful to know if they are all in the same subnet. If many of them are in the same subnet there's a good chance that a firewall is blocking their access to the WSUS server. Here's a SQL Query that should return the IP address of all computers with unknown patch status.
SELECT DISTINCT dbo.v_R_System.Name0, dbo.v_GS_NETWORK_ADAPTER_CONFIGUR.IPAddress0
FROM dbo.v_R_System
INNER JOIN dbo.v_GS_NETWORK_ADAPTER_CONFIGUR ON dbo.v_R_System.ResourceID = dbo.v_GS_NETWORK_ADAPTER_CONFIGUR.ResourceID
WHERE dbo.v_UpdateComplianceStatus_Unknown.ResourceID in
(select resourceid from dbo.v_UpdateComplianceStatus_Unknown) and (NOT (dbo.v_GS_NETWORK_ADAPTER_CONFIGUR.IPAddress0 = 'null'))
and dbo.v_GS_NETWORK_ADAPTER_CONFIGUR.IPAddress0 not like '0.0.0.0'
order by IPAddress0