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It is worth double checking to see if your voice text responses works. If it does it may be an issue with your account link between Amazon and fitbit. If it's not working it's more likely the microphone or the firmware that controls how the microphone processes input.
In my case the microphone isn't working at all. Ever since the most recent firmware update not only is my battery life less than a full day, but it appears the microphone isn't registering or processing my voice anymore. Alexa routines as well as voice response to text stopped working.