It appears that you have two planar elements to form the top and bottom of the assembly and a bunch of joists/rafters inside. However, when you select the whole object, there is also another element that fills the whole space between the top and bottom surfaces. I suspect that this is your problem. Unless you do as Oliver suggests, ArchiCAD will merge all the similar fills which seems to be the effect you are seeing.
Better still comment out, or turn off is some other way, the other element and the details should show up fine.
Erich
AC 19 6006 & AC 20
Mac OS 10.11.5
15" Retina MacBook Pro 2.6
27" iMac Retina 5K