My WiFi enabled air purifier was responding to Alexa voice commands, but returns unresponsive sometimes now. This occurs also when using the Alexa controls on my phone. It can take more than one try to work.
The mydyson app works ok, eliminating my network as the cause.
Voice control to all other devices work well. Eliminating Alexa as the cause.
This appears to be a fault at Dyson servers.