...and if that doesn't do it, turn off Cast Shadows on the ground plane and/or zoom in the light's ShadowCam on the objects that you want to cast shadows. Poser's Shadow Maps are generated from the view from each light's ShadowCam. If they are zoomed out too far, the shadowmap's antialiasing will blur out the shadows on objects that appear too small in the ShadowCam's field of view.