Saying either "Open Netflix" or "Launch Netflix" should load the app. Try restarting your Fire TV device. If it still will not launch, let's make sure your Fire TV is fully updated. Go to Settings > My Fire TV > About > Check for System Update.
I hope that helps! If it doesn't - are you only having an issue opening Netflix with this command, or is it happening to other apps as well? Let us know!
Yes. I had the same problem. I had to keep restarting the stick—then it worked. Then after a while Alexa would reply to a command like “tune to HGTV” and nothing would happen. Finally disconnect the HDMI extender and plugged the stick straight into the TV (although it stuck out). This fixed the problem. Try it and respond.