If you are using the Razer Kraken and VoiceAttack is not listening to you properly, you might want to try setting the option, 'disable acoustic echo cancellation' on the options screen. It seems as if the Kraken software/driver interfere with the speech engine instance. This fixes the issue for most.
Update - Some users have indicated that the Synapse software's, 'Active Mic Noise Cancellation' setting can cause the speech engine's recording device volume to drop out unexpectedly. Try disabling this setting as well if you continue to have issues.
Hope that helps!