This is re-post or echo of a post I wrote six months ago. The subject came up again in a couple emails.
I was positive I wrote about this in the past but I can't find a post in the pile of posts I've made since the end of 2004. I suppose it could be buried in a post at AUGI...
If a wall is less than six feet (or 2000mm, no not exactly equal to six feet) it will NOT use the Cut line weight, it will only show using the Projection line weight. You can "force" Revit to show it as Cut by setting the Top Constraint to the level above and using a negative Top Offset value to drop it back down to it's intended overall height. A bit obtuse but it works! [Added this: Chris mentioned in a comment, "you can set the Top Constraint to the same level as the Base Constraint and then use a positive value for your Top Offset value."]
I thought to post this (was thinking re-post but couldn't find a first) because it came up during a session at RTC (during Harlan Brumm's Troubleshooting class) and then was tweeted.
It is also documented in the current WikiHelp.
Keep in mind that if the height of the low wall is still lower than the cut plane of the view's view range setting it will not show using the cut lineweight, only the projection lineweight. The distinction of this post is for walls that are taller than the cut plane but not quite tall enough (six feet).
You can read the series of comments in my earlier post regarding Low Walls.