The items didn't works all in the same way (I haven't found a similar thread, if it was already signalled sorry and don't be afraid to lock this one).
For example, items from the "tend" selection would fly, but not the items on "little items" (sorry, I've setted my game with Italian language, if you need more specific infos just ask)
Just stay "outside", put some items from each class
You would have, in your preview, some objects that fall at ground (or over an other object between them and the ground) - others would stay at the given position.
It would be nice a 3D editor :)
The "problem" can be extend at the overlaps of the different items textures.
See the image for more infos
As a workaround, you can add this to the init of those items:
this enableSimulation false;
and they should not be affected by gravity anymore.