Successful Voice Command recognition also depends a lot on the quality of your Bluetooth radio, memory, and processor.
The HTC Prophet and latest Kaiser have decent bluetooth radios and get great voice recognition. Many other especially older devices weren't very good at transmitting and translating bluetooth audio.
So if you're not getting good results, there are plenty of possible things to blame!
Also, read the Help file to learn what commands it recognizes. Some commands support more than one way of saying it, and the other way may be easier for it to recognize.