Have you picked the light in the 'Shadow' rollout ?
Is the option 'Hide planes in viewport' disabled ? (it must be disabled to work with VRay)
Are you a using material based on 'Planes Material' ? (Forest needs 'Planes Material' to cast raytraced shadows)
If all these are ok, please send us the scene to email@example.com for checking. You may delete non-essential objects, we only need Forest, Planes and related objects (splines, lights, cameras and so).