Go to your Skyrim folder (Not Data) and open SkyrimEditor.ini. Under [Grass] is bAllowCreateGrass set to 1? If it's not there at all add it. If it is there, make sure it's set to one. (It will say bAllowCreateGrass=1)
Then open your mod and edit the ground some, making sure that all the grass is still there. That may potentially fix your issue.