OS? Graphics card? Sweet Home 3D version?

AMD graphics on windows: see this topic: https://www.sweethome3d.com/support/forum/viewthread_thread,11888_lastpage,yes (Update graphics driver)

On Linux: try the startup script SweetHome3D-Java3D-1_5_2 instead of the default SweetHome3D.

If none of these work first supply your system specifications and Sweet Home 3D version.