svaero:
I just played a bit with
Quoins 18 in the 3D and Elevation Windows and did not find a problem. You will need to post more specific information (a screen shot would help).
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David