Adding Cupla affects entire plan

[BB boy]

WELL, I made my first attempt doing it with DJP's You tube.....didn't work. I AM doing correctly, but it MUST have something to do with the already complete plan. The minute I go Build Floor>Blank Floor> open a Perspective camera view, the software kills my gables that have been there from the beginning, it sends the gable vents to the sky. Just don't understand it? If I star a new blank plan, and do the very same operations as the You Tube, it works beautifully. It is just when I go back to MY plan and try it over, the problem reappears. Getting frustrated!