Particle system doesn't affect part of mesh? I'm trying to create a grass effect on a small landscape I've created. My current approach is to use a hair particle system, and style the hairs to look like grass. I've got a simple particle system added, and adjusted a few settings (note: I haven't made the grass green yet. It's just grey for now), but for some reason the particles don't go on certain parts of the mesh:

You can see that to the left of the "lake" there is a lot of missing particles, in triangular areas. Although this screenshot was taken in rendered 3d view, it also has the same issue when I press the render button and do a "true" render. Here's some of the geometry:


Any ideas why it isn't completely covered with particles?
Make sure use modifier stack is checked under the emission rollout of the particle settings.