I don't know what the conditions are there but in all likelihood the plants have reached a balance as they are now. The grass that's struggling is probably going to be doing better than moss in that spot, otherwise the moss would probably already be there. You need to see what difference there is between where the moss currently is and where it isn't, and then try to make the conditions the same.

You might want to try contacting Moss Acres for advice and help. I read in the NYT that the owner and founder of Moss Acres got started in growing moss by converting his yard from grass to moss as an experiment that turned into a passion and business.

http://www.nytimes.com/2008/05/01/garden/01moss.html

http://www.mossacres.com/about_us.asp

Also try Annie Martin at Mountain Moss Enterprises, “Mossin’Annie”
http://mountainmoss.com/

Mountain Moss Enterprises

Mountain Moss Enterprises and Mossin’ Annie

Mountain Moss Enterprises and Mossin’ Annie