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, sun (though like shade more because of more moisture in shade), and a little nutrients does not hurt, but it generally comes from it surrounding. basically you can add nutrients to an area you want moss such as beer, or yogurt, or buttermilk. I like buttermilk. if spores in area, it should grow. you can take moss, and blend it with an ingredient mentioned above and paint it / spray it (depends on your mixture) where you want moss, it should grow. reason it may not...not enough moisture. basically just play with those scenarios. it does not rain in San Francisco for about 6 months, the moss dries up. as soon as the moisture is back, the moss thrives again. or you can just spray it with water to help it.