Speech recognition in iOS periodically fails - beep does not sound after hitting microphone button
The same thing happens to me periodically. It's so annoying when I can speak into the microphone and see the line wiggle around as if it is hearing what I'm saying but then when I finish it doesn't actually type any text for me. However, I just read online that sometimes you can fix it if you force close background apps (to free up memory) and also temporarily toggle into and out of Airplane Mode (to reset network connectivity). I haven't tried that myself yet.