Troubleshooting Audio Command Processing
If the voice recognition does not work as desired and Alexa often answers with “I didn’t understand you”, this can possibly be solved by adjusting the response time.
To do so, open the device manager in the Amazon Alexa App and select “Echo Auto”. Here you can activate the response time and the option “Fix truncated voice commands” under “Audio error handling”. In most cases a response time of 500 ms is ok, but in some cases an increase can help Alexa to understand voice commands better.
https://diysmarthomeplanet.com/echo-auto-setup/#Troubleshooting_Troubleshooting