I am relatively new to this myself so someone else may be able to provide more accurate information but usually think of it like this.
volume is the equivalent of the volume control of a physical amp
master volume is the volume level applied at the end
So I guess in a simplified scenario I would think that guitar volume, drive and volume would impact distortion levels and that master volume and your headphones/speaker volume would not. Of course, the amp you are modeling will make a big difference and anything else in the chain can also have an impact.
I would try turning the guitar volume up, turning the drive up, leaving the master volume moderate and use you speaker/headphones to adjust volume to taste. Also, make sure you are using a suitable amp model.