In this case you should use a smaller clump of grass. The billboard of your scene is too wide, and it is difficult to adjust the distribution at the edges.
Of course, using smaller objects it is necessary to increase the density, and the number of items grows. So you must balance between the size and density to get the best result. The right values depends of each scene.
I also suggest that you set the "Boundary Check" to "Size" in the Area options. On this mode, the boundary checking is more precise, using not only the position of the items, but also their size.
If that helps, the same issue is discussed here, although in this case using 3D grass. You will find a ready-to-use scene in the post:http://www.itoosoft.com/forum/index.php?topic=1105.0