Speech Recognition not working properly...

make sure your mic isn't covered. restart your computer and try it again. make a new way for it to listen. Make it stop listening all the time. make a new voice prompt before it listens. turn off voice prompt for it to listen.

Play around with the settings until they work for you. It's finicky to say the least. Try talking clearly like a Newsreporter. 😝

This solved it for me, also. I wanted to emphasize that you can use the Nuance non-built-in voices finally by choosing the voice under System Preferences->Dictation and speech, then deleting all speech files from ~/Library/Preferences. I also disabled Speech Recognition and re-enabled it, though perhaps that was unnecessary, and then everything worked. (Note: you may have to try speech recognition (say some commands) a few times before it will finally respond. I think it takes a couple passes to re-form the entries in the speech plists.)

Additional note: Not so oddly, a while after deleting the speech files all my preferences for Speakable Items had gone to default. I re-entered my settings (including Nuance voice Serena), and it kept working.