When you hit "Arm" in the mobile app, the Novi Hub will tell each Novi Cam to arm, and the app will show that the system is armed once all Novi Cams have responded that they are indeed armed.
If any Novi Cam is offline when you arm the system, the Novi Hub will wait up to 30 seconds for that Novi Cam to respond and then it will reflect that your other Novi Cams are armed. Try bringing your offline Novi Cam back online by either rebooting it, or putting new batteries into it. Or, you can remove the Novi Cam from your Novi Hub.
If you have multiple Novi Cams attached to one Novi Hub, it can take up to 10 seconds for all the Novi Cams to arm.