I have the same problem sometimes. It seemes to happen when two walls are parallell with the reference line touching the other (as in the image below). It often helps to have the reference line not touching the other wall. It's rather annoying since it's practical to have the reference linje following the other wall so you can change composite and easely trace the other wall. It also exports to IFC so it's not purely a bug in the view in Archicad.
Martin Isak Jansen
(I'm the one everyone at my company asks the tech questions)www.artec.no