Which browser, what hardware ? Some of the previews, in particular the wall thickness test and related "3Dtools", require WebGL support, which must be enabled in your browser settings (and require relatively modern OpenGL-capable graphics hardware and drivers).
Thanks I have preview in 3d tools, I have WebGL support. I only don't have preview when I want to put my designs in sets (mods) And there is no preview when I go to my shop
Ah, sorry, seems I did not quite understand. There is a known problem with the "Product sets" mod since shapeways introduced the new model codes several months ago, see this thread