Suzi doesn't musk me anymore either, they also might be getting used to your scent. Garters have a very acute sense of smell and can often pick up the scent of a mate or trails left behind by other snakes through the pheromones given off by their skin. They might possibly recognize our pheremones and then deem us as trustworthy or friendly. That would result in less musking as they grow and realize your not a threat but yet a source of food and water... and housekeeping lol

I always talk to them when I enter the room to let them be aware of my presence. Maybe they also learn to recognize the vibrations given off by your voice as with each tone there is a different vibe.