Create existing terrain from point cloud

I am trying to create an existing terrain from a point cloud.  The point cloud was created from an aerial LiDAR survey that contains topographic elevations and bathymetry.  For some reason only the topographic elevations (points above elev. 0) are imported into the terrain.  I am not using any filters on the data.  Any ideas as to why this is happening?  I am using Power InRoads SS4 version 08.11.09.872.  The same thing happened when trying to create the terrain direct from the LAS file.