Walls disappear when exporting to .dwg

In my Revit 2012 project I have several views set up that I use to export .dwg files. On one (and only one) of the views a handful of walls are no longer showing up in the exported file. There appears to be no logic or pattern in the problem - some entire walls are missing, some have small pieces missing, and others are fine. It seems like most of the missing walls are of one or two types, and I can't find any properties that are different for those missing walls. I've tried restarting and duplicating the view, and neither worked. Any ideas?