I don’t know what could be the problem… On one sketch you draw all the rectangles for the rooms and the outside line and the extrude the the ceiling height.
Then add the doors and windows openings.
Maybe 15 minutes in total?
I started drawing our house so I could play with it with FlowExpress. I am struggling with error in measurements. I started with the outside perimeter of the house and then made that a solid up to the main height a bit above ceiling height, then started adjusting for the rooms with lower ceilings. I then shelled that and started adding the walls between rooms with thin extrusions. Each room gets a sketch, but I have found that it doesn’t all add up, and I’m missing measurements. One of my other goals is to determine how much open space there is between some closets with the thought of perhaps enlarging one of the closets.
Good luck. If you start with good measurements it should be more straightforward.