When using FMOD_OUTPUTTYPE_WAVWRITER_NRT, you need to call System::update for any mixing to happen, this is the non-realtime (NRT) component of wav writer. Each call to update will produce 512 samples of audio output. Using FMOD_OUTPUTTYPE_WAVWRITER internally a thread is created that will do the writing in real-time.
The file will be written synchronously with each mixer update, make sure you call System::release at the end to properly terminate the output file with the correct file length.