If the agent is not able to collect data from Windows Update to check the update status, or the agent is unable to download the updates (both due to network problems at the client, proxy settings as example), the agent will not see any new update, so the device is displayed as "up to date" in the dashboard.

Please set an alert, when the agent is not able to pull windows update information or the updates itself, and why (you may have an exception which you could forward to the alert).