Hmm.. so I'm not sure (yet) if there is a report that will show exactly what you wanted (machines that have not received updates in x days).
However, there may be some useful info in the reporting section:
IF you know that the machines are WSUS clients and IF you have run (or have a recurring schedule to run) a WSUS Inventory, then the data collected there will populate some reports under WSUS Reports -> Windows Server Update Services Analytics folder.
You can then run the WSUS Server Computers Info report. Here are some sample results from that report showing "last synchronization time" and "last reported status time":
So, you could see if machines have been talking to WSUS with that one.
If you think you might have some machines in the domain that are NOT talking to the WSUS server (and therefore not getting updates from it), you check the policy settings for a specific machine by browsing to it and right-clicking to select Computer Explorer or, if it is easier, you can go to Administration and Reporting and then double-click Computer Explorer there to launch a dialog where you can type details about he machine to which you want to connect.
Once you are in Computer Explorer, click on the Windows Update Settings tab. When it pulls back data, sort by the Name field and then scroll down to Intranet WSUS Server service location and see if it is, in fact, pointing to your WSUS server.