I beleive it is harder to grow moss then grass. The soil needs to be acidic. Put some lime down. I have the same problem in an area that gets plenty of sun but the soil is too acidic for any grass to grow. This spring I plan on an all out attack on the moss I may spray a week lye solution on it. I will get a test kit first so I can tell what I am up against.The topsoil was added after a major septic tank job and I suspect it came from a peat bog not really a mineral soil, more like decaying leaves. It was very dark and fine.