There are a number of potential reasons for this:
-The Echo's microphone could be dirty, which would impact its ability to hear you clearly. Try cleaning the microphone with a cotton swab.
-If you've placed the Echo in a room with a lot of background noise, that could also be affecting its ability to hear you. Try moving it to a quieter location.
-It's also possible that the Echo's software could be outdated, which can impact its performance. To check for updates, open the Alexa app and go to Settings > Device Settings > Select your device > Check for Software Updates.