I am not a scientist here, but essentially, moss spreads via spores. If the spores are present and the conditions are right, moss grows. Moss needs water, and sun (though they like shade more due to moisture tends to remain longer in shady areas). A little nourishment does not hurt, but they are able to acquire it from their surroundings. Basically you can add nutrients to the area where you want moss to grow…good food includes beer, yogurt, or buttermilk. If moss spores are in area, moss should grow.
To help spread spores, you can take moss and blend it with a nourishment ingredient mentioned above then paint it or spray it where you want some moss and it should grow. A reason for growth failure using this technique is there is not enough moisture.
I have observed that it does not rain in San Francisco for about 6 months over the summer. Living moss dries up, and looks dead. However, as soon as the moisture is back, the moss will come back to life and thrive again.