Do you have a Draytek router by any chance - there is an issue with the default setting that needs tweaking.
The TCP WWW Timeout value is too low on Draytek routers for Harmony via Alexa to work consistently. The default value on my Draytek was 60 seconds, I saw a post from Logitech saying to try increasing it and since setting mine to 3600 secs mine has worked every time.