Voice commands not recognized

I've had my 2007 Prius for about 5 months and am really beginning to enjoy the voice nav features. I have read over and over all the possible voice commands for speaking destinations but I cannot get the system to recognize one or two of them. I keep trying to say "enter destination" but the system continues to say something like "that function is available in the scrolled map" or something similar to that. Why won't the system allow me to speak "enter destination"? Do I have to be in a specific part of the map or do some other cancel command before this one will work?