DIAGNOSIS The microphones are likely obstructed or the device is placed in a suboptimal location. The 5th Gen has a different microphone array.
FIX Clean the four microphone ports on the top rim with a dry, soft brush. Move the Dot away from walls, speakers, or under cabinets. Ensure it's on a solid surface, not on a vibrating shelf.
IF THAT DIDN'T WORK Background noise compensation may be off. In the Alexa app, go to Device Settings > your Echo > Sounds > Adaptive Sounds and toggle it off. Retrain the voice model.
HEADS UP The device performs a microphone self-test on boot. If the light ring doesn't illuminate blue when you say "Alexa," the microphone hardware may have failed.