How can I fix this?
My guess is that you have some flipped normals. Try viewing them with the Normals/Edit Normals modifiers and see if there are any problems. also, that green cone kind of looks planar - which means when rendered you'd naturally only be able to see one side because the normals can only point in one direction.
try searching the web on ways to make your planes be visible and have normals from/in both directions. some game engines have this functionality built in as an option.