jon:
You need to post a fuller description of your issue. Where (plan, section, 3D?) are you viewing the fill, and how (fill tool, part of another element?) is the fill being added? Images always 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