It may be because of one of the following few reasons:
If you turn off the Thermostat for more than 8 minutes, the sensor will lose connection with the Thermostat and will automatically go into standby mode. Press the Pairing Button on the Occupancy Sensor for 3 seconds.
The “Occupancy Detection Enabled” or “Sensor Enabled” is set to “No” at the Portal. Set both the “Occupancy Detection Enabled” and “Sensor Enabled” to “Yes”.
The sensor may not be immediately connected to the Thermostat when you have just added your sensor to the Portal. Wait for 1-2 minutes and check the “Status” for your Occupancy Sensor at the Portal. It should be “Connected”.
The active Mode of your Thermostat is "Sleep". Sensor is only working if the active Mode is "Home" or "Away".
The Thermostat is on Temporary/Permanent/Vacation Hold. Cancel the Hold so that the Thermostat is on Schedule mode.