Speech recognition problems...

Hi all, I'm having a strange problem with my mac taking commands with the speech recognition program in lion. it used to work fine but all of a sudden it doesn't respond to me, it can hear me because i can see the bars jumping up and down. I've tried changing the button etc but still no joy. I have a feeling it may of started when i changed the computer voice but I've tried changing it back and it still doesn't work. Any ideas?? thanx