Selection disapears Archicad 18 to SKP 15

[congellous]

So big model 180MB imported from BIM ArchiCAD

Latest OS X, SKP2015 fast computer

open model and make a selection of a number of floors - spin spin - selection goes blue and just before I can group it it disappears not the geometry just what I wanted to select

[congellous]

Archi Cad models come in with the lines hidden and no group structure so you can't see the selection or what you have grouped.

Still, can't work out why what you have selected becomes unselected by itself

[Rich O Brien]

Can you screengrab the issue?