Retry "Hello Thermostat" voice trigger phrase with a brief pause between words. You may want to move closer to thermostat and/or raise your voice slightly when retrying the voice trigger.
Please ensure your thermostat is connected to your WiFi network for voice control operation.
To improve voice trigger phrase recognition, you should adjust the voice sensitivity level. Go to Menu>Voice Control and select the Sensitivity option on your touch screen. A slider bar will appear which indicates your current sensitivity setting. You should adjust your sensitivity level to a higher number to help improve the recognition of the trigger command (Hello Thermostat). You may modify the voice control sensitivity multiple times to optimize your voice experience.