Reindex WSUS Database with below script,
UPDATE [sUSDB].[dbo].[tbComputerTargetDetail] SET [OSDescription] = 'Windows 8.1' WHERE [OSMajorVersion] = '6' AND [OSMinorVersion] = '3' AND [OldProductType] = '1' AND ([OSDescription] <> 'Windows 8.1' or [OSDescription] IS NULL)
Or check this tutorial