When you go to the Clients view and select 'all clients with a policy', notice the 'i' that appears to the right of the time period filter. The pop-up says: "This view will show you all clients that have a policy, regardless of when they last transferred data. The usage data is for the selected time period."
In other words, you should see all clients on the network that have a policy, even if they haven't had any activity in the last 30 days.