In this case, make sure that you follow the correct steps to trace the floor plan.
On the Tools tab, click Start Trace Wizard.
Select the story that you will start with and click Next.
Browse for your image file and open it into the program (jpg, png, or pdf)
Click Start Calibration to enter measurements of a known length of a wall
Go to the building tab and add your walls
When you are finished tracing your floor plans, you can toggle this view off or on using the Hide/Show Trace Image button
Repeat the process to add and trace floor plans for additional stories
You can also check the tutorial section of the program to know more about it: https://www.nchsoftware.com/design/tutorial.html