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