COPY-PASTE doesn't work under Mint 18

I made a drawing (walls and floors) and then tried to copy everything from level 1 to level 2. Unfortunately that wasn't possible.
- I tried CTRL-A then CTRL-C then CTRL-V without success.
- I tried select with mouse, then using the menu Copy and Paste without success.

Copy-Paste only works when selecting one or more items (but not ALL) at a time and copy-paste it to the new level.

NB: this is about walls and floors only, it's a design so far without furniture.

System: Mint 18, based on Ubuntu 16.04, SH3D version 5.5a

Hans