If you find that, even after extensive speech recognition training (3 session is the absolute minimum), specific words aren't recognized reliably or at all, you'll want to try adding those words to the speech recognition dictionary with a custom pronunciation.
Click the wrench icon on VoiceAttack's main window, and on the "Recognition" tab of the VoiceAttack options window click the "Utilities >" button, then choose "Add/Remove Dictionary Words"