I have a building design that was originally done using AC17. As time has gone by and AC has been upgraded (now using AC21 most current update) I also upgraded the file to be native in the current version (21 today). I wanted to show the design to a local company so I decided to look at the unit in 3D one floor at a time. To my surprise items on other floors were visible to which I determined that these items that should not be visible were linked to a floor different than their own. Please note that when viewing the complete model in 3D everything looks ok. As a result I decided to reference everything to their proper floor. Tedious but it had to be done. When I got to a specific floor/level the slab did not show in 3D. Attached the slab back to the proper floor in the proper location. When I went to view in 3D the slab when looking at the TOP or BOTTOM surface was clear yet the edge/side of the slab showed with the correct surface colour. I tried changing the top and bottom surfaces using the specifications box/menu but no luck (still invisible). I had just recently installed the Cadimage addon as instructed. Besides the clear slabs I notice some odd behaviour with some of the other slabs such as setting the surface to concrete light displayed as purple. Anyone got any ideas what might be happening?