A platform hole cannot touch the edge of the platform, extend past the edge of the platform, or be contained by multiple platforms.
A gap between floors is usually caused by modifying the Platform to this Line option in your Wall Type Definitions dialog so that it is on the exterior surface.
A given plan can have multiple floors but only one can be active for editing at a time but a reference floor can be superimposed on the working floor.
To make the program recognize the Open Below room make sure that at least one of the walls of this room is attached to another wall in the plan.
The Build House Wizard feature allows you to start a new floor plan by creating and arranging 2D room boxes and then converting these boxes into rooms.